This paper deals with defining leadership as one of the basic determinants of behavior in organizations. It points to the increasing importance of the discipline of organizational behavior (OB), which is devoted, as an interdisciplinary field of study, to better understanding and managing human side of the work. Three basic levels of analysis in OB are individual, group and organizational level. Leadership represents the key variable of organizational behavior, which is predominantly determined by personal individuals' traits and characteristics of the environment. Definition of the leadership from the project GLOBE was accepted, claiming that it is "the ability of an individual to influence, motivate and enable others to contribute toward the effectiveness and success of the organizations of which they are members". In second part of the paper, very influential theoretical differentiation between managers and leaders (and, of course between management and leadership) has been analyzed. According to this standing point, it is emphasized how good manager brings certain degree of predictability and order in organization, while successful leader initiates change, often to dramatic proportions. Nevertheless, overemphasizing the differences between them is often inappropriate, because concepts of leader and manager in above-mentioned sense represent more "ideal types" that almost never exist in reality in its "pure" forms. Last part of the text discusses the role of leadership in organizational transformation (OT). Very frequently assertion that key moment of OT is changing the culture of organization is cited and the role of leadership in three main phases of transformation is analyzed: "unfreezing", moving and "refreezing".