The paper introduces the present knowledge of the theory and technology of diffusion photon density wave (DPDW) which have been focus of the recent research.We discuss the relationship among various theories that have been presented to describe light diffusion in turbid medium such as tissues,and indicate the usefulness and limitations of these theories.Then we discuss diffusion theory and the relative time/frequency domain techniques in detail.Special attention is paid to the frequency domain technique and relevant systems for different applications.The new progress and prospect of the technology are presented.