The neostriatum is the larges* component of the basal ganglia, and the main recipient of afferents to the basal ganglia from the cerebral cortex and thalamus. Studies of the cellular organization of the neostriatum have focused upon the spiny projection neurones, which represent the vast majority of neurones, but the identity and functions of interneurones in this structure have remained enigmatic despite decades of study. Recently, the discovery of cytochemical markers that are specific for each of the major classes of striatal interneurones, and the combination of this with intracellular recording and staining, has revealed the identities of interneurones and some of their functional characteristics in a way that could not have been imagined by the classical morphologists. These methods also suggest some possible modes of action of interneurones in the neostriatai circuitry.