s book is intended to be used as a textbook and a reference to learn about singular perturbation methods and their use in applications. It presents a constructive approach which is primarily analytical, but which is also related to current efforts in numerical computation. The applications discussed are intended to be illustrative, so that the reader can go on to solve new problems. The presentation is closely related to current mathematical and applied literature, and it is written to be accessible to students of mathematics, engineering, and the sciences.