Apathy can be defined as a reduction of goal-directed behavior. It is frequently observed in psychiatric and neurological diseases, and presently assessed using clinical questionnaires. To understand the neural dysfunction underlying apathy, it is necessary to decompose this syndrome into elementary computational processes. A key distinction is that of costs and benefits: apathy can result either from hyposensitivity to potential rewards or from hypersensitivity to potential efforts. In this talk, I will present a behavioral paradigm that implements the conflict between effort and reward in humans, the crucial feature being that payoff is proportional to the energy expended. As suggested by model-based analyses of neuro-imaging and patient studies, such a paradigm might provide some insights into the brain mechanisms underlying normal and deficient motivation, as well as treatment effects. For instance, dopamine agonists appeared to increase reward attractiveness, whereas serotonin reuptake inhibitors seemed to alleviate effort costs.