Max/MSP and Pd are the data flow audio programs electronic musicians prefer for their rapid prototyping capabilities, graphical resemblance to analog synthesizer patching, and a wide variety of available synthesis and processing methods. However, a powerful element of these programs is surprisingly under-utilized: the ability to create (in C) new audio externals to process audio with sample-level precision. The advantage of writing externals in C is the ability to gain far greater control over the specification of new signal processing algorithms that are otherwise difficult or impossible to achieve with data-flow patching techniques. Additionally, externals coded in C can be considerably more CPU-efficient than the comparable algorithm implemented as a patch. Learning how to write externals for Max/MSP and Pd in C opens an entirely new world of creative possibilities for electronic musicians. This book guides the reader step-by-step through the process of designing externals, from concept through implementation. Twelve externals are presented, each revealing new sonic, musical, and programming possibilities. The multi-platform approach of this book supports the compilation of Pd externals on Linux, Mac OS X, and Windows, and the compilation of Max/MSP externals on Mac OS X and Windows. The CD-ROM contains complete code for all projects presented in the book. The Max/MSP externals are designed for use with Max 5; a supplementary chapter on the CD-ROM describes how to update Max 5 code to make use of 64-bit processing with Max 6.