Abnormal cognition is integral to our understanding of depression and mania according to diagnostic criteria, symptoms, and psychological models. In the present chapter, we highlight the use of neuro-imaging techniques and neurocognitive tests to investigate core cognitive abnormalities in patients, with a focus on affective processing bias, response to negative feedback, and decision-making. We also highlight the utility of cognitive tests as objective markers of brain function that may help elucidate aetiological contributions, monitor treatment response, and develop more effective treatment algorithms for mood disorders.