This book provides a single source for both students and advanced researchers on asymptotic methods employed in the linear problems of mathematical physics. It opens with a section based on material from special courses given by the author, which gives detailed coverage of classical material on the equations of mathematical physics and their applications, and includes a simple explanation of the Maslov Canonical Operator method. The book goes on to present more advanced material from the author's own research. Topics range from radiation conditions and the principle of limiting absorption for general exterior problems, to complete asymptotic expansion of spectral function of equations over all of space. This book serves both as a manual and teaching aid for students of mathematics and physics and, in summarizing for the first time in a monograph problems previously investigated in journal articles, as a comprehensive reference for advanced researchers.