This new reference is the first fully self-contained and unified monograph on the design and operation of undulators and free-electron lasers. It will be of use to all those working in free-electron laser research as well as for physicists and graduate students who need an introduction to the field. The book includes a convenient overview of early developments and general principles. The different models that are used to describe free-electron laser are presented, sensibly organized according to their range of applicability. Conceptual and mathematical constructs are covered, with an emphasis on obtaining practical results in a simple yet rigorous fashion. Interaction of the undulator with the driving accelerator is treated along with the laser cavity and the design of undulator magnets. A very useful overview of several typical experiments is also included.