Serotonin have been thought to play an important role in regulation of emotional states as well as decision making. Previously we have reported that the activity of the dorsal raphe serotonin neurons was increased by ketamine and olanzapine, promising therapeutic agents for treatment-resistant depression, as well as conventional antidepressants such as selective serotonin reuptake inhibitors and tricyclic antidepressants. It is, however, still unclear whether the activation of the dorsal raphe serotonin neurons is sufficient for eliciting antidepressant-like effects. Furthermore, identification of responsible serotonergic circuits for a variety of behaviors and disorders has been hampered by wide-ranging projection of serotonin neurons. To address these issues, we developed viral vectors capable of optogenetic manipulation of serotonergic neurons specifically. In this symposium, we would like to show our recent findings on the role of serotonin neurons in the regulation of mood and decision making.