The neostriatum is considered to be involved in the induction of purposeful movements or in the suppression of other movements through the internal segment of the globus pallidus (GP; the entopeduncular nucleus in the rodents) and the substantia nigra (SN) to which the striatal spiny neurons project. To understand how the striatum fulfills these functions, it is necessary to know the physiological and morphological characteristics of its constituent neurons. Aspiny interneurons in the striatum are considered to receive various excitatory inputs and to contribute importantly to determining whether spiny projection neurons fire or not. Both spiny and aspiny striatal cells have been shown to be heterogeneous in their physiological, chemical and connection characteristics. In this article, how these cell subtypes are organized in the local circuitry of the striatum and their physiological roles in the basal ganglia are discussed.