Drawing on the idea that humans aim to avoid wasting energy that is important for survival, motivational intensity theory postulates that task difficulty and success importance determine energy investment. Additionally, the theory makes predictions on how task characteristics moderate the relationship between task difficulty, success importance, and energy investment. In this article, I will show how the different predictions of motivational intensity theory relate to one another and to what extent they can be derived from the fundamental principle of energy conservation. I will also discuss the application of the theory to effort mobilization and cardiovascular reactivity. Specifically, I will discuss the additional assumptions and predictions that are necessary to link these applications to motivational intensity theory’s basic rationale, the energy conservation principle.