This article highlights the psychological process that determines the intensity of effort—the mobilization of resource for action execution—in performing cognitive tasks. I outline the logic of a theory-based psychophysiological approach. Arguing for a physiological measure of the intensity of exerted effort based on the definition of the effort construct, I present exemplary studies showing how specific responses in the cardiovascular system can reflect the mobilization of resources for action execution. Grounded in the principles of motivational intensity theory (Brehm & Self, 1989), that evidence shows that effort rises with subjective task difficulty as long as success is possible and the necessary effort is justified. Importantly, this means that people are fine with exerting high effort if that is justified and necessary. Rather than avoiding effort itself, people try to avoid the waste of effort. Differences with and implications for other approaches are discussed.