When people think about the concept of power, they often think about political leaders. In fact, Forbes's annual ranking of the most powerful leaders in the world usually ranks a number of top governmental officials and heads of state from different countries as their most powerful. This chapter begins by looking at how power has been operationalized over time. It describes what power actually does, both good and bad, to the power holder. In particular, it explains the importance of who the leader is when it comes to the outcomes of his or her power expression. Power can come from many sources. The sources can be structural, cognitive, from traits, and physical in nature. These different sources of power are evident in the "means" aspect of the definition of power, which suggests that power can be gained and enforced through a variety of mechanisms. When individuals possess power, they experience changes. The chapter details the cognitive, affective, behavioral, and neurochemical changes that can occur to power holders. It then explains what power does to leaders overall, taking these specific changes into account, thus setting up the discussion of whether powerful leaders tend to use their power for good or not. (PsycInfo Database Record (c) 2022 APA, all rights reserved)