This book provides a comprehensive treatment of both the theoretical underpinnings and the practical application of aircraft modeling based on experimental data, also known as aircraft system identification. The methods and algorithms explained in the book are implemented in a NASA software toolbox called SIDPAC (System IDentification Programs for AirCraft). SIDPAC is written in MATLAB, and is available by request from NASA Langley Research Center to U.S. citizens only. SIDPAC is composed of many different tools that implement a wide variety of practical approaches explained fully in the book. These tools can be readily applied to solve aircraft system identification problems.