This book provides a comprehensive overview of both the theoretical underpinnings and the practical application of aircraft modeling based on experimental data - also known as aircraft system identification. Much of the material presented comes from the authors' own extensive research and teaching activities at the NASA Langley Research Center and is based on real world applications of system identification to aircraft. The book uses actual flight test and wind tunnel data for case studies and examples, and should be a valuable resource for researchers and practicing engineers, as well as a textbook for postgraduate and senior-level courses. All aspects of the system identification problem - including their interdependency - are covered: model postulation, experiment design, instrumentation, data compatibility analysis, model structure determination, state and parameter estimation, and model validation. The methods discussed are used routinely for risk reduction during flight envelope expansion of new aircraft or modified configurations, comparison with wind tunnel test results and analytic methods such as computational fluid dynamics (CFD), control law design and refinement, dynamic analysis, simulation, flying qualities assessments, accident investigations, and other tasks. The book includes SIDPAC (System IDentification Programs for AirCraft), a software toolbox written in MATLAB[registered], that implements many methods discussed in the text and can be applied to modeling problems of interest to the reader.