The formation of the proper pattern of neuronal circuits during development is critical for the normal function of the vertebrate brain and for the survival of the organism. Circuit tracing studies spanning the past 100 years have revealed the beauty and exquisite intricacy of this pattern, which represents the most complex biological system known. In humans, aberrant circuit formation is a likely underlying cause of a wide variety of birth defects and neurological disorders, including autism, intellectual disability, and schizophrenia. Furthermore, future therapeutic approaches to restoring the function of damaged neural circuits will require a better understanding of the developmental constraints under which those circuits were originally assembled. For these reasons, elucidating the molecular mechanisms of neural circuit formation is a major goal of neurobiology today.