Leadership is and has been described as the “process of social influence in which one person can enlist the aid and support of others in the accomplishment of a common task”.
A definition more inclusive of followers comes from Alan Keith of Genentech who said "Leadership is ultimately about creating a way for people to contribute to making something extraordinary happen".
Leadership is one of the most relevent aspects of the organizational context.
Defining leadership has been challenging.
According to the late Jules Masserman, American psychoanalyst and former member of the faculty of Northwestern University medical school, leaders must fulfill three functions: the leader must provide for the well-being of the led, provide a social organization in which people feel relatively secure, and provide a set of beliefs.
Leaders must fulfill three functions: the leader must provide for the well-being of the led, provide a social organization in which people feel relatively secure, and provide a set of beliefs.
Students of leadership have produced theories involving traits, situational interaction, function, behavior, power, vision and values, charisma, and intelligence among others.
Trait theory tries to describe the types of behavior and personality tendencies associated with effective leadership.
Trait theory is probably the first academic theory of leadership.
Thomas Carlyle can be considered one of the pioneers of the trait theory, using such approach to identify the talents, skills and physical characteristics of men who arose to power.
Ronald Heifetz traces the trait theory approach back to the nineteenth-century tradition of associating the history of society to the history of great men.
Proponents of the trait theoryh usually list leadership qualities, assuming certain traits or characteristics will tend to lead to effective leadership.
Shelley Kirkpatrick and Edwin Locke exemplify the trait theory.
Shelley Kirkpatrick and Edwin Locke argue that "key leader traits include: drive, leadership motivation, honesty, integrity, self-confidence, cognitive ability, and knowledge of the business.
Drive is a broad term which includes achievement, motivation, ambition, energy, tenacity, and initiative.
Leadership motivation is the desire to lead but not to seek power as an end in itself.
self-confidence is associated with emotional stability.
According to Shelley Kirkpatrick and Edwin Locke, "there is less clear evidence for traits such as charisma, creativity and flexibility".
Although trait theory has an intuitive appeal, difficulties may arise in proving its tenets, and opponents frequently challenge this approach.
The "strongest" versions of trait theory see these "leadership characteristics" as innate, and accordingly labels some people as "born leaders" due to their psychological makeup.
In response to the criticism of the trait approach, theorists began to research leadership as a set of behaviors, evaluating the behavior of successful leaders, determining a behavior taxonomy and identifying broad leadership styles.
David McClelland, for example, saw leadership skills, not so much as a set of traits, but as a pattern of motives.
David McClelland claimed that successful leaders will tend to have a high need for power, a low need for affiliation, and a high level of what he called activity inhibition.
Kurt Lewin, Ronald Lipitt, and Ralph White developed in 1939 the seminal work on the influence of leadership styles and performance.
The managerial grid model was developed by Robert Blake and Jane Mouton in 1964 and suggests five different leadership styles, based on the leaders' concern for people and their concern for goal achievement.
Situational theory also appeared as a reaction to the trait theory of leadership.
Social scientists argued that history was more than the result of intervention of great men as Carlyle suggested.
Some theorists started to synthesize the trait and situational approaches.
The authoritarian leadership style, for example, is approved in periods of crisis but fails to win the "hearts and minds" of their followers in the day-to-day management; the democratic leadership style is more adequate in situations that require consensus building; finally, the laissez faire leadership style is appreciated by the degree of freedom it provides, but as the leader does not "take charge", he can be perceived as a failure in protracted or thorny organizational problems.
The authoritarian leadership style, for example, is approved in periods of crisis but fails to win the "hearts and minds" of their followers in the day-to-day management.
The democratic leadership style is more adequate in situations that require consensus building.
The the laissez faire leadership style is appreciated by the degree of freedom it provides, but as the leader does not "take charge", he can be perceived as a failure in protracted or thorny organizational problems.
The Fiedler contingency model bases the leader’s effectiveness on what Fred Fiedler called situational contingency.
The Fiedler contingency model results from the interaction of leadership style and situational favorableness.
The Fiedler contingency model defined two types of leader: those who tend to accomplish the task by developing good-relationships with the group, and those who have as their prime concern carrying out the task itself.
According to Fiedler, there is no ideal leader.
Both task-oriented and relationship-oriented leaders can be effective if their leadership orientation fits the situation.
When there is a good leader-member relation, a highly structured task, and high leader position power, the situation is considered a "favorable situation".
Fiedler found that task-oriented leaders are more effective in extremely favourable or unfavourable situations, whereas relationship-oriented leaders perform best in situations with intermediate favourability.
Those who tend to accomplish the task by developing good-relationships with the group are relationship-oriented.
Those who have as their prime concern carrying out the task itself task-oriented.
Victor Vroom, in collaboration with Phillip Yetton and later with Arthur Jago developed a taxonomy for describing leadership situations, taxonomy that was used in a normative decision model where leadership styles where connected to situational variables, defining which approach was more suitable to which situation.
The taxonomy for describing leadership situations was a novel approach because it supported the idea that the same manager could rely on different group decision making approaches depending on the attributes of each situation.
The taxonomy for describing leadership situations was later 
referred as situational contingency theory.
The path-goal theory of leadership was developed by Robert House and was based on the expectancy theory of Victor Vroom.
According to House, the essence of the path-goal theory is "the meta proposition that leaders, to be effective, engage in behaviors that complement subordinates' environments and abilities in a manner that compensates for deficiencies and is instrumental to subordinate satisfaction and individual and work unit performance.
The path-goal theory identifies four leader behaviors, achievement-oriented, directive, participative, and supportive, that are contingent to the environment factors and follower characteristics.
In contrast to the Fiedler contingency model, the path-goal model states that the four leadership behaviors are fluid, and that leaders can adopt any of the four depending on what the situation demands.
The path-goal model can be classified both as a contingency theory, as it depends on the circumstances, but also as a transactional leadership theory, as the theory emphasizes the reciprocity behavior between the leader and the followers.
The situational leadership model proposed by Hersey and Blanchard suggests four leadership-styles and four levels of follower-development.
Functional leadership theory is a particularly useful theory for addressing specific leader behaviors expected to contribute to organizational or unit effectiveness.
Functional leadership theory argues that the leader’s main job is to see that whatever is necessary to group needs is taken care of; thus, a leader can be said to have done their job well when they have contributed to group effectiveness and cohesion.
While functional leadership theory has most often been applied to team leadership, it has also been effectively applied to broader organizational leadership as well.
In summarizing literature on functional leadership, it is observed there are five broad functions a leader performs when promoting organisation's effectiveness.
The five broad functions a leader performs when promoting organisation's effectiveness are environmental monitoring, organizing subordinate activities, teaching and coaching subordinates, motivating others, and intervening actively in the group’s work.
A variety of leadership behaviors are expected to facilitate these functions.
In initial work identifying leader behavior, Fleishman observed that subordinates perceived their supervisors’ behavior in terms of two broad categories referred to as consideration and initiating structure.
Consideration includes behavior involved in fostering effective relationships.
Initiating structure involves the actions of the leader focused specifically on task accomplishment.
The transactional leader is given power to perform certain tasks and reward or punish for the team’s performance.
It gives the opportunity to the manager to lead the group and the group agrees to follow his lead to accomplish a predetermined goal in exchange for something else.
Power is given to the manager to evaluate, correct and train subordinates when productivity is not up to the desired level and reward effectiveness when expected outcome is reached.
The transformational leader motivates its team to be effective and efficient.
Communication is the base for goal achievement focusing the group on the final desired outcome or goal attainment.
The transformational leader leader is highly visible and uses chain of command to get the job done.
Transformational leaders focus on the big picture, needing to be surrounded by people who take care of the details.
The transactional leader is always looking for ideas that move the organization to reach the company’s vision.
Leadership can be perceived as a particularly emotion-laden process, with emotions entwined with the social influence process.
In an organization, the leaders’ mood has some effects on his group.
Group members with leaders in a positive mood experience more positive mood than do group members with leaders in a negative mood.
The leaders transmit their moods to other group members through the mechanism of emotional contagion.
Mood contagion may be one of the psychological mechanisms by which charismatic leaders influence followers.
Group affective tone represents the consistent or homogeneous affective reactions within a group.
Group affective tone is an aggregate of the moods of the individual members of the group and refers to mood at the group level of analysis.
Groups with leaders in a positive mood have a more positive affective tone than do groups with leaders in a negative mood.
Public expressions of mood impact how group members think and act.
When people experience and express mood, they send signals to others.
Leaders signal their goals, intentions, and attitudes through their expressions of moods.
Expressions of positive moods by leaders signal that leaders deem progress toward goals to be good.
In research about client service, it was found that expressions of positive mood by the leader improve the performance of the group, although in other sectors there were other findings.
Beyond the leader’s mood, his behavior is a source for employee positive and negative emotions at work.
The leader creates situations and events that lead to emotional response.
Certain leader behaviors displayed during interactions with their employees are the sources of these affective events.
Leaders shape workplace affective events.
Since employee behavior and productivity are directly affected by their emotional states, it is imperative to consider employee emotional responses to organizational leaders.
Emotional intelligence is the ability to understand and manage moods and emotions in the self and others, contributes to effective leadership in organizations.
Leadership is about being responsible.
In the past, some researchers have argued that the actual influence of leaders on organizational outcomes is overrated and romanticized as a result of biased attributions about leaders.
It is largely recognized and accepted by practitioners and researchers that leadership is important, and research supports the notion that leaders do contribute to key organizational outcomes.
In order to facilitate successful performance it is important to understand and accurately measure leadership performance.
Job performance generally refers to behavior that is expected to contribute to organizational success.
Campbell identified a number of specific types of performance dimensions; leadership was one of the dimensions that he identified. 
There is no consistent, overall definition of leadership performance.
Many distinct conceptualizations are often lumped together under the umbrella of leadership performance, including outcomes such as leader effectiveness, leader advancement, and leader emergence.
For instance, leadership performance may be used to refer to the career success of the individual leader, performance of the group or organization, or even leader emergence.
An organization that is established as an instrument or means for achieving defined objectives has been referred to as a formal organization.
Formal organization specifies how goals are subdivided and reflected in subdivisions of the organization.
Divisions, departments, sections, positions, jobs, and tasks make up this work structure.
The formal organization is expected to behave impersonally in regard to relationships with clients or with its members.
According to Weber's definition, entry and subsequent advancement is by merit or seniority.
Each employee receives a salary and enjoys a degree of tenure that safeguards him from the arbitrary influence of superiors or of powerful clients.
The higher his position in the hierarchy, the greater his presumed expertise in adjudicating problems that may arise in the course of the work carried out at lower levels of the organization.
It is this bureaucratic structure that forms the basis for the appointment of heads or chiefs of administrative subdivisions in the organization and endows them with the authority attached to their position.
In contrast to the appointed head or chief of an administrative unit, a leader emerges within the context of the informal organization that underlies the formal structure.
The informal organization expresses the personal objectives and goals of the individual membership.
In prehistoric times, man was preoccupied with his personal security, maintenance, protection, and survival.
Now man spends a major portion of his waking hours working for organizations.
Leaders emerge from within the structure of the informal organization.
A leader is a person who influences a group of people towards a specific result.
Leaders are recognized by their capacity for caring for others, clear communication, and a commitment to persist.
An individual who is appointed to a managerial position has the right to command and enforce obedience by virtue of the authority of his position. 
In the absence of sufficient personal competence, a manager may be confronted by an emergent leader who can challenge his role in the organization and reduce it to that of a figurehead.
whoever wields personal influence and power can legitimize this only by gaining a formal position in the hierarchy, with commensurate authority.
Leadership can be defined as one's ability to get others to willingly follow.
Every organization needs leaders at every level.
Management involves power by position.
Leadership involves power by influence.
Managers administer; leaders innovate.
Managers ask how and when; leaders ask what and why.
Managers focus on systems; leaders focus on people.
Managers do things right; leaders do the right things.
Managers maintain; leaders develop.
Managers rely on control; leaders inspire trust.
Managers have short-term perspective; leaders have long-term perspective.
Managers accept the status-quo; leaders challenge the status-quo.
Managers have an eye on the bottom line; leaders have an eye on the horizon.
Managers imitate; leaders originate.
Managers emulate the classic good soldier; leaders are their own person.
Managers copy; leaders show originality.
managers concerned themselves with tasks while leaders concerned themselves with people.
Effective leaders create and sustain competitive advantage through the attainment of cost leadership, revenue leadership, time leadership, and market value leadership.
Managers typically follow and realize a leader's vision.
The difference lies in the leader realising that the achievement of the task comes about through the goodwill and support of others, while the manager may not.
This goodwill and support originates in the leader seeing people as people, not as another resource for deployment in support of "the task". 
The manager often has the role of organizing resources to get something done.
A leader has the role of causing others to follow a path he or she has laid out or a vision he or she has articulated in order to achieve a task.
an organization might have the overall task of generating profit, but a good leader may see profit as a by-product that flows from whatever aspect of their vision differentiates their company from the competition.
Leadership does not only manifest itself as purely a business phenomenon.
Many people can think of an inspiring leader they have encountered who has nothing whatever to do with business: a politician, an officer in the armed forces, a Scout or Guide leader, a teacher, etc.
management does not occur only as a purely business phenomenon.
we can think of examples of people that we have met who fill the management niche in non-business organisationsNon-business organizations should find it easier to articulate a non-money-driven inspiring vision that will support true leadership.
In contrast to individual leadership, some organizations have adopted group leadership.
In group leadership more than one person provides direction to the group as a whole.
Others may see the traditional leadership of a boss as costing too much in team performance.
In some situations, the maintenance of the boss becomes too expensive, either by draining the resources of the group as a whole, or by impeding the creativity within the team, even unintentionally.
A common example of group leadership involves cross-functional teams.
A team of people with diverse skills and from all parts of an organization assembles to lead a project is a cross-functional team.
Leaders who demonstrate persistence, tenacity, determination and synergistic communication skills will bring out the same qualities in their groups.
Good leaders use their own inner mentors to energize their team and organizations and lead a team to achieve success.
Sanskrit literature identifies ten types of leaders.
Defining characteristics of the ten types of leaders are explained with examples from history and mythology.
Aristocratic thinkers have postulated that leadership depends on one's blue blood or genes: monarchy takes an extreme view of the same idea, and may prop up its assertions against the claims of mere aristocrats by invoking divine sanction.
Contrariwise, more democratically-inclined theorists have pointed to examples of meritocratic leaders, such as the Napoleonic marshals profiting from careers open to talent.
Reliance on intelligence alone results in rebelliousness.
Exercise of humaneness alone results in weakness.
Fixation on trust results in folly.
Dependence on the strength of courage results in violence.
Excessive discipline and sternness in command result in cruelty.
When one has all five virtues together, each appropriate to its function, then one can be a leader.
Leadership is a matter of intelligence, trustworthiness, humaneness, courage, and discipline.
Other historical views of leadership have addressed the seeming contrasts between secular and religious leadership.
At certain stages in their development, the hierarchies of social ranks implied different degrees or ranks of leadership in society.
Thus a knight led fewer men in general than did a duke; a baronet might in theory control less land than an earl.
In the course of the 18th and 20th centuries, several political operators took non-traditional paths to become dominant in their societies.
Noam Chomsky and others have criticized the very concept of leadership as involving people abrogating their responsibility to think and will actions for themselves.