This book introduces the basic concepts of MathLink and explains how to extend Mathematica using MathLink. MathLink provides a simple and powerful way to write programs which communicate with Mathematica and it provides access to a wide range of network resources. Writing network programs is the most demanded skill in the Internet age and MathLink provides a sophisticated level of network programming to the non-programmer, scientist, engineer, and student. The text assumes that the reader is already familiar with Mathematica. Source code for a range of practical examples - ranging from elementary to advanced - is examined in detail, allowing the reader easily to adapt the code to suit their own needs. The purpose of this book is to show how to write MathLink programs and how to apply MathLink in a variety of situations. A CD-ROM accompanies the book, which includes the code accompanying the examples and the application CodeWarrior Lite.