This book is the continuation of a series of the author's books on the reduction of nonlinear control systems to a simpler form and application of reduction to control problems. Here the author considers the reduction to decomposition into systems of lesser dimension. The approach centres on the immersion of control systems into some differential geometric categories. Within the framework of the category the decomposition is based on factorization. By way of example, a control problem of practical interest (invariance to perturbations) is examined to illustrate the application of decomposition.