How neural circuits form and function has been an area of scientific research for over a century. In my doctoral thesis, I aim to add to our understanding of these processes at a fundamental level. The common fruit fly, Drosophila melanogaster, is an excellent model to examine the basic principles and molecules that govern neural development. Here, I utilize the cell surface protein families, the Dprs and DIPs, to study specific steps of neural circuit assembly: axonal pathfinding and synaptic growth. Specifically, I show that Dpr10 functions in axonal guidance in the embryonic peripheral nervous system. In addition, Dpr10 and its binding partner DIP-α inhibit synaptic growth, thereby contributing to the specification of synaptic arbor size in a motor neuron and target dependent manner. To understand how these processes occur at the molecular level, I collaborated with Viola Nawrocka to examine the biochemical properties of the Dprs and DIPs and demonstrated that most are tethered to the outer leaflet of the cell membrane through GPI anchors; this modification has implications for their subcellular localization and potential signaling mechanisms. Together, this body of work examines how cell surface proteins contribute to neural circuit assembly and development.