Reward is essential for motivating goal-directed behaviour. Impairment in the processing of reward is therefore a promising candidate for understanding apathy which has been defined as a loss of motivation and a quantitative reduction of goal-directed behaviour. This chapter employs the recently updated Research Domain Criteria framework for positive valence systems to provide an overview of reward system functions that have been associated with apathy, including reward anticipation, reward consumption, learning and prediction error, value representation, and integration of effort. For each construct, the concept and the measures on the behavioural and neural level are discussed. The chapter then provides examples from the schizophrenia literature on the association of apathy with these functions and gives a transdiagnostic perspective on the role of reward system dysfunction.