This article highlights the systematic impact of experienced and implicit affect on the intensity of mental effort. The key argument is that both consciously experienced affect and implicitly activated affect knowledge can influence responses in the cardiovascular system reflecting effort intensity by informing individuals about task demand—the key variable determining resource mobilization. According to the motivational intensity theory, effort rises with experienced demand as long as success is possible and the necessary effort is justified. Twenty-five years of programmatic research have provided clear evidence that both consciously experienced affect and implicitly activated affect knowledge systematically influence the intensity of effort. Importantly, affect's impact on effort is moderated by task context variables, like objective task difficulty, incentive, and other general boundary conditions.