Our goal is to make robotics more accessible to casual users by reducing the domain knowledge required in designing and building robots. Towards this goal, we present an interactive computational design system that enables users to design legged robots with desired morphologies and behaviors by specify- ing higher level descriptions. The core of our method is a design optimization technique that reasons about the structure and motion of a robot in a coupled manner to achieve user-speci ed robot behavior and performance. We are in- spired by the recent works that also aim to jointly optimize robot's form and function. However, through eficient computation of necessary design changes, our approach enables us to keep user-in-the-loop for interactive applications. We evaluate our system in simulation by starting with initial user designs that are physically infeasible or inadequate to perform the user-desired task. We then show optimized designs that achieve user-speci cations, all while ensur- ing an interactive design  ow.