This encyclopaedic book describes the developments of the last years in the area of asymptotic methods for linear ODEs and systems in the real and complex domain. Basically all main results and methods are given. Almost every known asymptotic formula is referred to. Written in a style readable for the nonspecialist in the area, the book is a guide to the extensive literature developed recently in the field. It is a much needed exposition and a comprehensive source of information for students and researchers in mathematics as well as in mechanics and physics.