As motion sensors have become more advanced, gesture-control systems have become more popular in gaming and everyday appliances. However, in existing systems, gestures are predefined by designers or pattern-recognition experts. Such predefined gestures can be inconvenient for specific users in specific environments. Hence, it would be useful to provide end users the flexibility to design and customize gestures to satisfy their own needs. In this paper, we present a system that allows end users to design and customize gestures interactively. A key challenge is that arbitrary user-defined gestures can be difficult for the computer to recognize reliably. A gesture may be too similar to frequent unintentional moves, too difficult to distinguish from other gestures, and/or too difficult to perform consistently. Hence, our system first evaluates the userdefined gesture and then gives feedback on its appropriateness to guide the user in the design of appropriate gestures. A user study demonstrated that users were able to design more appropriate gestures with such guidance than without it.