'Form follows function' is an architectural philosophy attributed to the great American architect Louis Sullivan, and later taken up by the Bauhaus movement. It stresses that the form of a building should reflect its function. Neuroscientists have used the converse of this dictum to learn the functions of neural circuits, believing that if we study neural architecture, it will lead us to an understanding of how neural systems function. New tools for studying the structure of neural circuits are being developed, so it is important to discuss what the old techniques have taught us about how to derive function from the form of a neural circuit.