This rigorous treatment prepares readers for the study of differential equations and shows them how to research current literature. It emphasises nonlinear problems and specific analytical methods. Based on a Brown University course in applied mathematics, this rigorous and demanding treatment focuses on specific analytical methods. It emphasises nonlinear problems, acquainting readers with problems and techniques in ordinary differential equations. The material is presented in a manner that prepares students for informed research of differential equations, teaching them how to be more effective in studies of the current literature. In addressing the applied side of the subject, the text devotes considerable attention to specific analytical methods common to applications. Introductory chapters offer necessary background material by reviewing basic facts of analysis and covering the general properties of differential equations. Topics include two-dimensional systems, linear systems and linearization, perturbations of noncritical linear systems, simple oscillatory phenomena and the method of averaging, and behavior near a periodic orbit. Additional subjects include integral manifolds of equations with a small parameter, periodic systems with a small parameter, alternative problems for the solution of functional equations, and the direct method of Liapunov. Exercises appear at the end of each chapter, and the appendix contains a convenient reference for almost every periodic functions.