In the first few years of life, humans tremendously expand their behavior repertoire and gain the ability to engage in complex, learned, and reward-driven actions. Similarly, in the few weeks after birth, mice gain the ability to perform sophisticated spatial navigation, forage independently for food, and to engage in reward reinforcement learning. Our laboratory seeks to uncover the mechanisms of synapse and circuit plasticity that permit new behaviors to be learned and refined. We are interested both in the developmental changes that occur after birth that make learning possible as well in the circuit changes that are triggered by the process of learning. We examine these processes in the cerebral cortex, the hippocampus, and the basal ganglia, crucial structures for the processing of sensory information, for associative learning and spatial navigation, and for goal-direct locomotion. In order to accomplish these studies, we rely heavily on optical approaches to examine and manipulate synapses and circuits in relatively intact brain tissue and we design and build microscopes as necessary for our research. Studies are typically performed in brain slices or awake-behaving mice and utilize a variety of genetic, biochemical, and electrophysiological approaches. Lastly, we use these same approaches and the knowledge gained from the study of normal circuit development to uncover perturbations of cell and synapse function that may contribute to human neuro-psychiatric disorders, including autism, Parkinson’s disease, and Alzheimer’s disease.