A crucial set of decisions in digital musical instrument design deals with choosing mappings between parameters controlled by the performer and the synthesis algorithms that actually generate sound. Feature-based synthesis offers a way to parameterize audio synthesis in terms of the quantifiable perceptual characteristics, or features, the performer wishes the sound to take on. Techniques for accomplishing such mappings and enabling feature-based synthesis to be performed in real time are discussed. An example is given of how a real-time performance system might be designed to take advantage of feature-based synthesis's ability to provide perceptually meaningful control over a large number of synthesis parameters.