Intrinsic motivation is widely viewed as a major force behind performance and creativity, and much scholarly attention has been paid to the positive effects of intrinsic motivation. However, while existing research has examined how intrinsic motivation in one task affects performance in that task, little research has examined how intrinsic motivation in one task affects performance in one's other tasks in a multiple-task environment. This is an important question as workplaces are increasingly becoming a multiple-task environment where one task's dynamics are influenced by another task's dynamics. In this dissertation, I explore the nature of the cross-task effects of intrinsic motivation. The main hypothesis is that intrinsic motivation in one task has a curvilinear relationship with performance in one's other tasks such that when intrinsic motivation in the focal task is medium, it helps performance in one's other tasks whereas when intrinsic motivation in the focal task is high, it hurts performance in one's other tasks. Under medium intrinsic motivation in the focal task, spillover of energy savings and positive affect prevails between the focal task and other tasks, but under high intrinsic motivation in the focal task, contrast and goal shielding effects prevail blocking the motivation and attention for the other tasks. Across three studies (one laboratory experiment and two field studies), I find that intrinsic motivation in a task indeed has curvilinear effects on performance in one's other tasks as well as overall performance variance across tasks, especially with high polychronicity. The theory and findings of this dissertation enhance our understanding of the complexities of intrinsic motivation and advance knowledge about motivation and performance management.