Interactive systems, from Java applets to virtual reality, benefit from sound. Sound reminds users of current state, acknowledges actions they take, and alerts them to unexpected events. This presentation covers both the mechanics and the aesthetics of interactive sound: how to choose "instruments" and then how to compose for them.  For instrumentation, low- and high-level multiplatform sound libraries are compared;different kinds of sound files and how to get them are also covered. For composing, common roles of these sounds are outlined: how sound interacts with visuals, speech versus non-speech, spatialization ("3D"), and effective combining or layering of sounds.