Speckle arises from interference of many waves with random phase relationship. In spite of its random appearance its statistical properties are independent of surface roughness of diffusers and depend only on macroscopic parameters of optical systems under usual conditions of observation. In this article we first discuss the size distribution of speckle from physical viewpoints. Then we describe the dynamic behaviors of speckle that are caused by displacement and deformation of diffuse surfaces as well as by change in optical system. Finally we overview applications of these properties of speckle to measurement of displacement and deformation.