This book introduces the concept of unsteady aerodynamics and its underlying principles. The author provides the readers with a comprehensive review of the fundamental physics of free and forced unsteadiness, the terminology and basic equations of aerodynamics ranging from incompressible flow to hypersonics. The book also covers modern topics related to the developments made in recent years, especially in relation to wing flapping for propulsion. The book is written for graduate and senior year undergraduate students in aerodynamics and also serves as a reference for experienced researchers. Each chapter includes ample examples, questions, problems and relevant references. This 3rd edition includes a new chapter about unsteady applications related to the thrust optimization, aerodynamic stability and trim because there has been much progress in unsteady applications of the flapping wing technology. In addition, further material is presented in Appendix for evaluating the stability derivatives so that no derivation of equations is left incomplete but not overdone in the text.