This issue of Current Topics in Behavioral Neuroscience focuses on the neuropeptide orexin (hypocretin) and brings together scientists from around the world who will provide a timely discussion of how this peptide regulates behavior. This is a fast-moving field, and with the incorporation of novel technologies, new breakthroughs are likely to continue. For example, the use of optogenetic approaches has enabled the identification of the role of orexin-containing neurons in arousal states, critical for higher order functioning. From a clinical perspective, genetic polymorphisms in...