This chapter deals with the psychological process that determines effort intensity in instrumental behavior. According to motivation intensity theory, effort should be proportional to experienced task difficulty as long as success is possible and justified and low when success is impossible or excessively difficult, given the available benefit. When task difficulty is unspecified or unknown, effort should be proportional to the importance of success. We report a program of experimental studies that have operationalized effort intensity as cardiovascular reactivity during task performance and used multiple manipulations of variables influencing subjective task difficulty (e.g., performance standards, ability, mood) and the amount of justified effort (e.g., material incentive, instrumentality, evaluation). The empirical evidence is in clear support of the principles of motivation intensity theory and challenges a number of other theoretical accounts. Directions for future research are discussed.