This volume is intended for researchers in engineering and applied mathematics. It can also be used as a textbook for graduate students dealing with non-linear systems and control theory. After a self-contained treatment of the differential-geometric prerequisites, the book deals with controllability and observability properties of nonlinear systems, as well as various ways to obtain input-output representations. Problems of transforming nonlinear systems into simpler forms are discussed, including the feedback linearization problem. The disturbance and input-output decoupling problem are treated in detail, as well as some aspects of feedback stabilization, and interconnection and inversion of nonlinear systems. Emphasis is put on fundamental notions as (controlled) invariant distributions and submanifolds, together with algorithms to compute the required feedbacks. Extensions of these methods to other synthesis problems are indicated in the exercises at the end of each chapter. Special attention is paid to mechanical nonlinear control systems, and finally the theory is extended to general continuous-time and discrete time systems. Numerous examples and exercises illustrate the main results of the book.