A professor in Stanford University's Center for Computer Research in Music and Acoustics is the author of this online book on digital filters. The material is mainly geared toward musicians, but it can be useful for anyone learning about digital signal processing. Available in draft form as of March 2003, the book contains equations, theorems, and principles of filter design spread across many chapters. Several MATLAB utilities and C++ implementations are also given. The only shortcoming of the Introduction to Digital Filters is its online presentation. If the material was condensed into fewer sections rather than being scattered across hundreds of Web pages, or if it could be downloaded as one large document, it would be much easier to read and follow.