Abstract: Reward motivates animal behaviors, produces the feelings of pleasure, and guides learning and memory formation. Reward processing involves a group of interconnected neural structures. Although dopamine neurons in the midbrain ventral tegmental area (VTA) receive much focus, whether and how neurons in the dorsal raphe nucleus (DRN) contribute to reward processing remain elusive. The DRN represents the major source of serotonin in the forebrain and contains also glutamate, GABA, and dopamine neurons. Here I will summarize recent evidences indicating that the DRN contributes to reward processing in a cell type-specific manner. First, I will present evidences indicating that DRN serotonin neurons and glutamate neurons encode reward signals (sucrose, food, sex, and social interaction) and are activated by various drugs of abuse that are associated with pleasure. Second, aversive stimuli activate DRN GABA neurons and lateral habenula neurons that are known to project to the DRN. Third, DRN dopamine neurons encode saliency and play an important role in memory expression. These results indicate that the DRN contributes to various aspects of reward processing through different neuron types associated with distinct neurotransmitter phenotypes. Finally, I will present our recent method development for brain-wide reconstruction of neuronal morphology and how this method reveal heterogeneous morphological features of serotonin neurons and dopamine neurons.