This book introduces the theory and research methods of partially coherent optical, points out the significance of this study, analyzes the basic theories involved in partially coherent light, and explains the complete coherence of laser is not a necessary condition for good directivity, and partially spatially coherent optical sources can also produce the same far-field intensity distribution as lasers. This type of beam with good directivity but only partial spatial coherence is called a Gaussian Schell model beam, which can be used as a physical model to describe multimode lasers under certain conditions. This book is applicable to the large number of engineers and technicians involved in wireless optical communication and college teachers. It can also be used as a teaching material for graduate students and senior undergraduates in related fields.