Shared leadership is a leadership style that broadly distributes leadership responsibility, such that people within a team and organization lead each other. It has frequently been compared to horizontal leadership, distributed leadership, and collective leadership and is most contrasted with more traditional "vertical" or "hierarchical" leadership that resides predominantly with an individual instead of a group.

Shared leadership can be defined in a number of ways, but all definitions describe a similar phenomenon: team leadership by more than just an appointed leader. Below are examples from researchers in this field:

Shared leadership is also commonly thought of as the "serial emergence" of multiple leaders over the life of a team, stemming from interactions among team members in which at least one team member tries to influence other members or the team in general. While the definition clearly has several variants, they all make the fundamental distinction between shared leadership and more traditional notions of hierarchical leadership. As Pearce, Manz and Sims (2009) summarize, all definitions of shared leadership consistently include a "process of influence" that is "built upon more than just downward influence on subordinates or followers by an appointed or elected leader." Nearly all concepts of shared leadership entail the practice of "broadly sharing power and influence among a set of individuals rather than centralizing it in the hands of a single individual who acts in the clear role of a dominant superior." Therefore, shared leadership is an emergent team property of mutual influence and shared responsibility among team members, whereby they lead each other toward goal achievement. It differs from team leadership, team processes and team work in that shared leadership describes a set of cooperatively oriented cognitions, attitudes, and actions through which team members convert member inputs to team outputs.

Though a relatively new phenomenon in the literature, the concept of shared leadership can actually be traced back several centuries. In a 2002 paper, David Sally noted that shared leadership was present even in the early days of Republican Rome. Indeed, during those ancient times, Rome "had a successful system of co-leadership that lasted for over four centuries. This structure of co-leadership was so effective that it extended from the lower levels of the Roman magistracy to the very top position, that of consul." (Sally, 2002) Despite such early iterations of the practice, however, most of the scholarly work on leadership has still been predominantly focused on the study of leadership in its hierarchical form. Leadership is conceived around a single individual – the leader – and how that person inspires, entices, commands, cajoles and controls followers. Research on shared leadership instead departs from the notion that leadership may well be studied as a collective phenomenon, as activities involving several individuals beyond the formally appointed manager.

There are some earlier conceptualizations of shared leadership. In 1924, Mary Parker Follet wrote that "one should not only look to the designated leader, but one should let logic dictate to whom one should look for guidance" (as cited by Crainer, 2002, p. 72). Along similar lines, Gibb, in 1954, wrote that, "Leadership is probably best conceived as a group quality, as a set of functions which must be carried out by the group." Despite these early nods toward group leadership, the formalized construct of shared leadership did not become more developed and experimentally explored until recently. Current research suggest that shared leadership forms may imply significant advantages at individual-, team-, organizational- and societal levels.

The shift in this scholarly paradigm might partly be explained by looking at the rise of studies on teamwork. Teamwork is becoming increasingly important in the workplace literature as many organizations recognize the benefits that teamwork can bring. Thus, organizations consider it important to investigate team effectiveness and the elements that increase this. Leaders have been pointed to as critical factors in team performance and effectiveness; some have even gone as far as to say they the most important ingredient for team effectiveness. Additionally, problems associated with team leaders are often cited as the primary reason for failures of work involving teams.

With the complexity and ambiguity of tasks that teams often experience, it is becoming more apparent that a single leader is unlikely to have all of the skills and traits to effectively perform the necessary leadership functions. Shared leadership has been identified as the optimal model of leadership when the knowledge characteristics of interdependence, creativity, and complexity are encountered. Thus, shared leadership is becoming increasingly popular in teams, as multiple team members emerge as leaders, especially when they have the skills/knowledge/expertise that the team needs.

There are two main ways that most researchers measure the existence and extent of shared leadership in a team: Ratings of the team's collective leadership behavior and Social Network Analysis. A less common technique of measuring shared leadership is with the use of behaviorally anchored rating scales.