Stellar speckle interferometry is a technique for obtaining diffraction-limited resolution of stellar objects despite the presence of the turbulent atmosphere which limits the resolution of conventional long-exposure pictures to approximately one arc second. It was invented by Labeyrie [7.1] and the first results using the 5 m Hale telescope at Mount Palomar were presented in 1972 [7.2]. Speckle interferometry can be regarded as a method of finding the modulus of the degree of coherence in the far field of an incoherent source and is essentially the same as a method suggested independently by Asakura et al. [7.3] for the determination of the spatial coherence of light emitted by laboratory sources. There is also a very close relationship between stellar speckle interferometry and some of the information processing and engineering applications of speckle patterns described in Chapters 5 and 6.