This chapter explores that dopamine (DA) has long been known to be a critical modulator of striatal processing of cortical and thalamic signals carried by glutamatergic synapses on the principal neurons of the striatum – medium spiny neurons (MSNs). DA regulation of these neurons is important for a wide array of psychomotor functions ascribed to the basal ganglia such as habit learning and the control of serial movement. In spite of its significance, an understanding of the physiological principles underlying MSN regulation has developed slowly. It reviews that major obstacle to unraveling the DA puzzle in the striatum has been the lack of homogeneity in the MSN class; there are at least two major subsets of MSNs that differ in their expression of DA receptors. These subsets cannot be readily identified on the basis of their somatodendritic morphology or electrophysiological properties. Both cell types are imbedded in a rich neuronal network involving both MSNs and interneurons that is modulated by DA. DA receptors are primarily found in dendrites that are inaccessible with electrodes, making direct study of their actions on glutamatergic signaling and dendritic excitability difficult. argely focuses on what is known about how DA modulates postsynaptic properties that influence glutamatergic synaptic events and their integration by MSNs in the dorsal striatum. Only the actions of the principal DA receptors in this region (D1, D2 receptors) are discussed.