This chapter reviews the basic anatomy of the SN and the morphological and electrophysiological characteristics of substantia nigra (SN) DA neurons and the control of these neurons by gamma-aminobutyric-acid (GABA)ergic inputs. The normal functioning of nigrostriatal DA neurons is crucial to a large array of behaviors ranging from voluntary motor function to higher cognitive processes. This remarkable variety of functions is even more impressive when one considers how few SN DA neurons there are relative to the striatum. It suggests that while the cellular mechanisms capable of generating the different firing patterns seen in vivo are intrinsic, afferent input is required to manifest the different firing patterns. The most numerous afferents to nigral DA neurons are GABAergic, and manipulation of GABA receptors on nigrostriatal neurons, principally GABAA receptors, produces dramatic effects on firing pattern, and more modest effects on firing rate, in vivo. The GABA afferent that seems to be most efficacious at modulating the firing pattern of nigrostriatal neurons comes from the axon collaterals of the substantia nigra pars reticulata (SNr) projection neuron. Many or most of the afferents to SN contact both the DA and the GABA neurons, providing the basis for a complex series of mono- and polysynaptic responses to both excitatory and inhibitory afferents.