Joshua R. Sanes, PhD, director of the Center for Brain Science at Harvard University, has been a pioneer in the study of the formation of synapses, the communication junctions between neurons. Early in his career, while studying the neuromuscular junction, the synapse where motor neurons transmit signals to muscle fibers, he demonstrated for the first time that some of the signals used to organize the synapse were contained in the extracellular matrix that surrounds the muscle fiber. That groundbreaking discovery marked a paradigm shift in how scientists view synaptic formation and organization. In a series of elegant experiments, Sanes then went on to identify and characterize many of the molecular factors within the extracellular matrix that help direct the formation of the neuromuscular junction. In more recent years, Sanes has turned his attention to the synaptic organization of neural circuits in the retina, which form the basis for visual processing. He has elucidated many of the molecules within those circuits that promote specificity, the ability of individual neurons to form synapses with specific subsets of presynaptic and postsynaptic partners. These findings have led to transformative new ideas about the functional architecture of the central nervous system.