The serotonergic (5HT) system plays a key role in modulating behaviors, such as appetite and anxiety and has been implicated in many human disorders of mood and mind. Recent studies have begun to identify the signaling molecules and transcriptional cascades governing 5HT neuron development in the hindbrain. Already at early stages, local differences in requirements of 5HT neuron development have become apparent. These studies point toward cryptic heterogeneity amongst 5HT neurons and suggest that 5HT neuron determination and differentiation may be more flexible and less absolute biologic processes than might have been expected. Ultimately, the intrinsic heterogeneity and environmental sensitivity of 5HT neruons may help explain the variability observed in some human behavioral disorders, such as autism spectrum disorder, and the less predictable behavioral consequences of fetal alcohol syndrome.