This book is devoted to the asymptotic theory of differential equations. Asymptotic theory is an independent and important branch of mathematical analysis that began to develop at the end of the 19th century. Asymptotic methods' use of several important phenomena of nature can be explained. The main problems considered in the text are based on the notion of an asymptotic space, which was introduced by the author in his works. Asymptotic spaces for asymptotic theory play analogous roles as metric spaces for functional analysis. It allows one to consider many (seemingly) miscellaneous asymptotic problems by means of the same methods and in a compact general form. The book contains the theoretical material and general methods of its application to many partial problems, as well as several new results of asymptotic behavior of functions, integrals, and solutions of differential and difference equations. Audience: The material will be of interest to mathematicians, researchers, and graduate students in the fields of ordinary differential equations, finite differences and functional equations, operator theory, and functional analysis.