The thalamus (Greek for “inner chamber”) is a large, egg-shaped mass of gray matter derived from the diencephalon of the developing brain (see eTable 9.1). The most significant role of the thalamus is as a synaptic relay for pathways projecting to the cerebral cortex. However, the thalamus also acts as a gatekeeper to prevent or enhance information transfer, depending on the behavioral state. Sensory, motor, limbic, and modulatory signals from behavioral and arousal circuits all have synaptic relays within the thalamic nuclei.