This book is intended to provide the reader with a guide to the issues involved with creating powerful and reliable add-ins for Excel. With years of use, many people build up the experience and understanding needed to create custom functions for Excel in C and C++. However, given the very limited books and resources available, this can be a largely trial-and-error process. The motivation in writing this book is to create something I wish I had had through the years: a coherent explanation of the relevant technology, what steps to follow, what pitfalls to avoid, and a good reference guide. With these things at your side, writing.