Major depressive disorder (MDD) is a common and debilitating illness. Impairments in reward processing may underlie the symptom of anhedonia (a lack of interest or enjoyment in typically enjoyable activities) in depression. This thesis aimed to further our understanding of reward processing in depression using a multidisciplinary approach, with a focus on risk for and vulnerability to the disorder. Specifically, the first part of this thesis investigated event-related potential (ERP) indices of reward-related feedback processing that have been associated with risk for depression, by examining their aetiology and association with depressive symptoms throughout adolescence in a longitudinal population-based twin sample. The findings indicate that these ERP indices of feedback processing are heritable and that genetic influences remain largely stable throughout adolescence. Unique environmental influences also contributed to these ERP indices of feedback processing and were largely age-specific during early-to-mid adolescence, becoming more stable from mid-to-late adolescence. However, there was a lack of a robust association between ERP indices of feedback processing and depressive symptoms, and the ERPs and depressive symptoms did not predict each other over time. The second part of this thesis focused on investigating specific components of reward processing, namely reward enjoyment and motivation, and their association with vulnerability to MDD. A task was developed to assess motivation to obtain rewards and used to investigate reward motivation in a sample of participants with remitted MDD (rMDD) and a healthy control comparison group. Despite no differences in rMDD and healthy control participants’ enjoyment of rewards, rMDD participants had reduced motivation to obtain rewards. Higher levels of depressive symptoms were associated with lower motivation to obtain rewards across the total sample. These findings indicate that reduced reward motivation may be associated with vulnerability to MDD. Collectively, these results contribute to our knowledge of behavioural and neurophysiological measures of reward processing in depression.