Most neurons of the striatum, approximately 95% in rodents, consist of GABAergic spiny projection neurons that form the major inputs and the only outputs of the nucleus. The remainder comprises several types of GABAergic interneurons that have been classified on the basis of electrophysiological properties, the expression of various calcium-binding proteins, neuropeptides or enzymes, and/or synaptic connectivity. These classification schemes have this far resulted in the identification of 10 major classes of GABAergic interneurons: a parvalbumin-expressing fast-spiking interneuron, a calretinin-expressing interneuron, two different neuropeptide Y–expressing interneurons, four subtypes of tyrosine hydroxylase-expressing interneurons, a fast-adapting interneuron, and a recurrent interneuron. This chapter will review the current state of knowledge of the anatomy and neurophysiology of striatal GABAergic interneurons, as well as their interactions with striatal cholinergic interneurons, spiny projection neurons, and cortical and thalamic afferents. The physiological properties of some of the GABAergic interneurons and their newly discovered interconnections suggest a heretofore-unknown complexity, diversity, and specialization of striatal GABAergic interneuronal function.