Orexin neurons are localized in the lateral hypothalamus and regulate many functions including sleep-wake states. Substantial number of neurotransmitters and neuromodulators has been proposed to influence orexinergic system. Glutamate, as the major excitatory amino acid neurotransmitter in the hypothalamus, was shown to mediate orexin neurons in the regulation of wakefulness and feeding. Glutamate is readily present in the lateral hypothalamus, and glutamate receptors are expressed by the neurons of this region. Glutamate agonists initiate excitatory postsynaptic currents in orexin neurons, and this can be blocked by specific antagonists of the glutamate receptors. It is reported that both NMDA and non-NMDA receptors contribute the glutamatergic neurotransmission which affects orexinergic functions. Glutamatergic axon terminals are demonstrated to make contacts with the orexin neurons, as revealed by the presence of vesicular glutamate transporter proteins in the terminals, and these contacts were ultrastructurally confirmed to establish synapses on orexin neurons. This chapter reviews the literature on the glutamatergic regulation of orexin neurons including the data from our laboratory.