The brain reward circuit has a central role in reinforcing behaviors that are rewarding and preventing behaviors that lead to punishment. Recent work has shown that the lateral habenula is an important part of the reward circuit by providing 'negative value' signals to the dopaminergic and serotonergic systems. Studies have also suggested that dysfunction of the lateral habenula is associated with psychiatric disorders, including major depression. Here, we discuss insights gained from neuronal recordings in monkeys regarding how the lateral habenula processes reward-related information. We then highlight recent optogenetic experiments in rodents addressing normal and abnormal functions of the habenula. Finally, we discuss how deregulation of the lateral habenula may be involved in depressive behaviors.