Leadership is one of the most salient aspects of the organizational context. However, defining leadership has been challenging. In reviewing the leadership literature Stogdill argued that “there are almost as many definitions of leadership as there are persons who have attempted to define the concept” (Stogdill, 1974, p. 259). Even though leadership is a term that is commonly used, defining leadership in specific terms can prove difficult likely leading to such a large number of definitions. Despite the multitude of leadership definitions, Zaccaro and Klimoski (2001) argued there are several common elements that transcend the many available definitions. Specifically, leadership involves a) processes and proximal outcomes that contribute to the organizational objectives, b) the application of non-routine influence, and c) is contextually defined and caused. Proximal outcomes that leaders could facilitate in the pursuit of achieving organizational objectives could include developing organizational commitment among subordinates. Non-routine influence implies that leaders must to have discretion in their actions and that their behavior should differ from influence provided through organizational routines. Finally, leadership needs to be considered with respect to the context in which it is occurring. One example is examining how leadership changes across levels of the organization. More broadly, leadership refers to organizing collective effort in the pursuit of solving problems facing the group (Kaiser, Hogan, & Craig, 2008). Thus, leadership includes social problem solving (Mumford, 1986; Mumford, Zaccaro, Harding, Jacobs, & Fleishman, 2000) and setting direction in social domains (Jacobs & Jaques, 1987), often to achieve collective action or organizational adaptation (Mumford et al., 2000; Yukl, 2006). Overall, it is important to note that leadership necessitates the presence of followers and it is inherently discretionary (Jacobs & Jaques, 1990)—without people to lead or the element of choice, leadership cannot truly be exerted. For a more thorough of comparisons between definitions of leadership as well as a summary of different styles of leadership please refer to reviews by Gary A. Yukl - Yukl (2006), Avolio, Sosik, Jung, and Berson (2003), Avolio, Walumbwa, and Weber (in press), and Den Hartog and Koopman (2002).  Do Leaders Matter?
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 (Meindl & Ehrlich, 1987). Despite these assertions however, 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 (Day & Lord, 1988; Kaiser, Hogan, & Craig, 2008). Identifying the relationship between leadership and organizational outcomes often becomes more difficult because of the manner in which leadership performance is often measured and that organizational outcomes are rarely accounted for (Kaiser et al., 2008).  Leadership Performance
The criterion space with regard to leadership has been muddied by varying conceptualizations and operationalizations of leadership outcomes. Many distinct constructs are often lumped together under the umbrella of leadership performance, including outcomes such as leader effectiveness, leader advancement, and leader emergence (Kaiser et al., 2008). While these constructs may be related, they are different outcomes and their inclusion should depend on the applied/research focus. As in discussions of performance more broadly (discussed in more detail below), it is important to distinguish between performance and effectiveness. That is, performance reflects behavior, while effectiveness implies the assessment of actual organizational outcomes (see Campbell, 1990 for a more detailed...