The glutamatergic subthalamic nucleus (STN) is a key node in the cortico-basal ganglia-thalamo-cortical circuit. As a component of the hyperdirect and indirect pathways, the STN is important for constraining movement, thought, and emotion through synaptic excitation of GABAergic basal ganglia output neurons. The STN also forms a reciprocally connected network with the GABAergic external globus pallidus that converges onto common basal ganglia output neurons. Abnormally correlated, rhythmic activity in this network is critical for the symptomatic expression of both Parkinson's disease and impulse control disorders, such as obsessive–compulsive disorder. The STN is therefore a clinically significant target for therapeutic deep-brain stimulation in these diseases. In this chapter, I will review the morphological, synaptic, and integrative properties of STN neurons and the relationship of their activity patterns in vivo to both normal and abnormal behavior.