This book presents modern functional analysis methods for the sensitivity analysis of some infinite-dimensional systems governed by partial differential equations. The main topics are treated in a general and systematic way. They include many classical applications such as the Signorini problem, the elastic-plastic torsion problem and the visco-elastic-plastic problem. The "material derivative" from which any kind of shape derivative of a cost functional can be derived is defined. New results about the wave equation and the unilateral problem are also included in this book, which is intended to serve as a basic reference work for the algorithmic approach to shape optimization problems.