Neurons of the cerebral cortex must precisely wire together to allow the flow of information that mediates cognition. Aberrations in the development of neural circuits contribute to the pathogenesis of neurodevelopmental disorders. My dissertation focuses on a family of neuronal cell adhesion molecules known as the gamma-protocadherins (-Pcdhs). Using mouse genetics, I have identified their functions in regulating multiple key steps in cortical circuit formation, and elucidated molecular mechanisms through which they act.