With the constantly increasing popularity of human multitasking, it is crucial to know why people engage in simultaneous task performance or switch between unfinished tasks. In the present article, we propose that multitasking behavior occurs when people have multiple active goals; the greater their number, the greater the degree of multitasking. The number of currently considered goals is reduced where one goal's significance overrides the others, reducing the degree of multitasking. We tested these hypotheses in a series of six studies in which we manipulated either goal activation or goal importance and investigated how this affected the degree of multitasking. The results showed that the more active goals participants actively entertained, the more likely they were to plan to engage in multitasking (Studies 1 and 5), and the more often they switched between tasks (Study 2). They also multitasked more under high interruption condition assumed to activate more goals than low interruption condition (Study 3). Further, we demonstrated that the degree of multitasking was significantly decreased by reducing the number of simultaneously considered goals, either via increasing the relative importance of one of the goals (Study 4) or via inducing greater commitment to one of the goals through a mental contrasting procedure (Study 5). Study 6, carried out in an academic context, additionally showed that the importance of a class-related goal negatively predicted media multitasking in class. The results thus show that goal activation is the underlying mechanism that explains why people multitask. (PsycInfo Database Record (c) 2022 APA, all rights reserved).