The importance of orexin in the regulation of sleep/wakefulness states, especially in the maintenance of wakefulness, was initially revealed by animal studies which showed the relationship between orexin and narcolepsy. Familial narcolepsy of dogs was shown to be caused by functionally-null mutations in the orexin 2 receptor (OX2R or hcrtr2) gene (Lin et al. 1999). At the almost same time, orexin gene disruption in mice was shown to cause narcolepsy (Chemelli et al. 1999). Orexin neuron-ablated (orexin/ataxin-3-transgenic) mice, and OX1R/OX2R double-deficient mice were subsequently showed to have the same phenotype with orexin −/− mice Willie et al. 2003), characterized by behavioral arrests that are similar to cataplexy, occasional direct transitions to REM sleep from wakefulness, and highly fragmented sleep-wake cycles, all of which are important elements of narcolepsy. These phenotypes have strong parallels to the human narcolepsy. In this chapter, I will discuss animal models of narcolepsy.