This chapter presents passion as one of the key elements leading to extreme behavior and outcomes. It first defines extremism as an infrequent phenomena entailed by an underlying motivation of high intensity or magnitude. The second section introduces the concept of passion and the predominant theory on passion, the Dualistic Model of Passion. According to this model, passion is a strong inclination for an important, meaningful, and self-defining activity in which one invests time and energy. It posits that there are two types of passion, harmonious and obsessive, that, given their specific characteristics, respectively lead to moderate and extreme consequences. The next two subsections uncover the role of passion in extreme interpersonal (e.g., violent activism, romantic and sexual behaviors, aggression, hatred, and dehumanization) and intrapersonal (e.g., addiction, burnout, and health problems) behaviors and outcomes. Overall, this chapter reveals that obsessive passion leads to a number of extreme interpersonal and intrapersonal consequences, whereas harmonious passion is related to less extreme outcomes such as mainstream and peaceful behaviors, positive relationship quality, and healthy behaviors. Finally, the last section offers some conclusions on passion and extremism.