It was after the advent of the laser and its applications in holography that the attention of research workers was attracted by the speckle effect which is produced when a diffusing object is illuminated in coherent light. The very particular properties of speckle and notably its fineness have led to the development of new techniques. A record of the speckle pattern on a high resolution photographic plate acts as an ordinary diffuser and most of the applications are derived from the interference phenomena produced by these diffusers. These phenomena have been known a very long time and the earliest observations were made by Newton towards the end of seventeenth century [5.1]. The duke of Chaulnes [5.2] discovered accidently how to increase considerably the visibility of the interference effects. Young [5.3], Her-schel [5.4] and many other physicists also studied these phenomena but Burch [5.5] was the first to use the interference fringes produced by two identical diffusers for measuring the deformations of a wave-front. It was after the pioneer work of Leith and Upatnieks on holography [5.6] that the effects of speckle were observed and their use in different techniques started developing. In what follows, we limit ourselves to phenomena related directly to speckle effect and do not consider its applications in holography.