There are many kinds of signal models to consider for musical instruments and audio effects. This book emphasizes physical signal models, that is, signal models derived from mathematical physical descriptions of acoustic instruments or electronic effects. To set a more general context, some of the more successful non-physical signal models are mentioned below. After that, we will consider only physics-based models in this book. The next volume [#!SASP!#] in the music signal processing series discusses spectral signal models, which are models based on the encoding of sound by the ear (the ``receiver'') as opposed to the mechanics of sound production (the ``source''). In all cases, a good signal model must have the property that it generates the desired sound in response to intuitive controls by the performing musician.