Dopamine (DA) is a key modulator of action selection and habit learning. The striatum has long been thought to be a major locus of DA action in this process. Although all striatal cell types express G protein–coupled receptors for DA, the effects of DA on medium spiny projection neurons (MSNs) understandably have received the most attention. In the two principal classes of MSN, DA receptor expression diverges, with direct-pathway MSNs robustly expressing D1 DA receptors and indirect-pathway MSNs expressing D2 DA receptors. In the last couple of years, there have been significant advances in our understanding of how DA modulates striatal circuits underlying learning, and how therapeutic interventions intended to normalize disordered DAergic signaling can go awry. This chapter highlights some of the major advances in these areas.