The intrinsic ability of an animal to adapt its behavior and achieve reward is fundamental to survival. Reward-guided behaviors elicit distributed activity across the brain, recruiting cortical and subcortical brain structures such as the prefrontal cortex (PFC), striatum, ventral tegmental area (VTA), and others. Recent advances in techniques for optical physiology have been transformative in our understanding of the brain’s reward system. The ability to measure and manipulate the activity of specific neurons during reward-guided behavior is beginning to shed light on the functional roles for genetically and/or anatomically defined neuronal populations. Here, we first provide an overview of imaging techniques enabling such studies, with an emphasis on measuring cellular and subcellular neuronal signals with two-photon microscopy using genetically encoded sensors for calcium and neurotransmitters like dopamine. We then describe how recent studies have applied these techniques to subcortical (dopamine system and striatum) and cortical (prefrontal cortex) systems of reward processing. Although this chapter is not meant as an exhaustive review of the literature, we highlight areas of inquiries where novel optical tools have provided important new data that have been used to both test old hypotheses and generate novel insights about the circuit organization of the brain reward system.