In recent years there has been a resurgence of interest in the study of delay differential equations motivated largely by new applications in physics, biology, ecology, and physiology. The aim of this monograph is to present a reasonably self-contained account of the advances in the oscillation theory of this class of equations. Throughout, the main topics of study are motivated by applications as diverse as the populations of blowflies, logistic equations in ecology, the survival of red blood cells in animals, integro-differential equations, and the motion of the tips of growing plants. The authors begin by reviewing the basic theory of delay differential equations including the fundamental results of existence and uniqueness of solutions and the theory of the Laplace and z-transforms. Consequently, little prior knowledge of the subject is required other than a firm grounding in the main techniques of differential equation theory. As a result this book provides an invaluable reference to the recent work both for mathematicians and for all those whose reseach includes the study of this fascinating class of differential equations.