Brehm's motivational intensity theory has been a fruitful conceptual framework for research on effort during the last three decades. Researchers have used the theory to address various effort-related phenomena, like the impact of ability, affect, and fatigue on effort mobilization. In this chapter, we provide an overview of development in the last 10 years focusing on research that has addressed (1) the energy conservation principle, (2) ability and fatigue effects, and (3) the impact of mood, dysphoria, and primed affect. We point out that most of the research has supported the predictions of the theory and its extensions and applications. However, we also elaborate on empirical findings that do not fit the theory and discuss open questions that need to be addressed in future research.