In the present monograph, I have collected some of the results which my cooperators and I have obtained in the last few years. The book is divided into seven chapters. Chapter 1 introduces some preliminary materials, including and some results from calculus of finite differences which are taken from the monograph of Agarwal(2000). Chapter 2 provides some results in the oscillation of first order delay difference equations. Chapter 3 discusses oscillation, nonoscillation and asymptotic classification of nonoscillatory solutions of second nonlinear difference equations. Chapter 4 investigates the oscillation and existence of nonoscillatory solutions of higher order neutral delay difference equations. Chapter 5 deals with oscillation for delay difference equations with continuous variables. Chapter 6 concerns oscillation and nonoscillation of functional equations. The last chapter, Chapter 7 studies oscillation and existence of nonoscillatory solutions of delay partial difference equations.