What 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 an extensive program of research that has operationalized effort intensity as cardiovascular reactivity during task performance and used multiple manipulations of variables influencing subjective task difficulty (e.g., performance standards, instrumentality, ability, fatigue, mood, depressive symptoms, implicit affect, implicit and biological aging) and the amount of justified effort (e.g., material incentive, instrumentality, needs, personal and social evaluation, mortality salience). In this second edition of this handbook, we focus on recent empirical evidence for the principles of motivation intensity theory and discuss challenges for other theoretical accounts.