Leadership is defined as “a process whereby intentional influence is exerted over other people to guide, structure, and facilitate activities and relationships in a group or organization” (Yukl GA, Leadership in organizations. Pearson, p. 2, 2013). As “[l]eading people effectively is a tremendous challenge, a great opportunity, and a serious responsibility” (Nahavandi A, The art and science of leadership. Pearson, p. xv, 2006), leadership is one of the most heavily researched phenomena in social sciences. Over the years, the leadership field has evolved with incorporating a range of concepts and theories from different disciplines (e.g., psychology, sociology, anthropology, organizational and political sciences), painting a clearer picture of the leadership paradigms and allowing us to explain the antecedents, outcomes, circumstances, and mechanisms of leadership. This chapter will provide a thorough overview of leadership theories and research. Specifically, this chapter begins by introducing early leadership theories (the trait, behavioral, and contingency approach to leadership). The chapter then turns to reviewing existing research on new leadership models (LMX, transformational leadership, ethical leadership, shared leadership, and abusive supervision), followed by a discussion of gender and cultural differences in existing leadership research. Finally, this chapter provides a brief overview of power, influence tactics, and organizational politics.KeywordsTrait approachBehavior approachSituational contingencyLeader-member exchange (LMX)Transformational leadershipEthical leadershipShared leadershipAbusive supervisionGenderCross-culturePowerInfluencePolitics