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The habenula is an epithalamic structure of the DDC linking forebrain to midbrain regions.The habenula regulates monoaminergic systems including those for dopamine and serotonin.Lateral habenula neurons encode negative reward prediction error and aversive stimuli.Sites within the DDC support operant responding for brain stimulation reward.Optogenetic activation of LHb projections to the midbrain produces behavioral avoidance.The dorsal diencephalic conduction system (DDC) is a highly conserved pathway in vertebrates that provides a route for the neural information to flow from forebrain to midbrain structures. It contains the bilaterally paired habenular nuclei along with two fiber tracts, the stria medullaris and the fasciculus retroflexus. The habenula is the principal player in mediating the dialogue between forebrain and midbrain regions, and functional abnormalities in this structure have often been attributed to pathologies like mood disorders and substance use disorder. Following Matsumoto and Hikosaka seminal work on the lateral habenula as a source of negative reward signals, the last decade has witnessed a great surge of interest in the role of the DDC in reward-related processes. However, despite significant progress in research, much work remains to unfold the behavioral functions of this intriguing, yet complex, pathway. This review describes the current state of knowledge on the DDC with respect to its anatomy, connectivity, and functions in reward and aversion processes.