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.