This paper reviews the models of the forward problem, which are analytic solution, numerical solution and the statistic solution. Especially, a case of the Monte Carlo (MC) method when a bioluminescence source in multi-spectral mode is embedded in the tissue is introduced. In the field of optical imaging, according to different imaging modalities, the demand for imaging quality and the extracted information, there are three major categories: continuous waves, time-domain, and frequency-domain. Here not only the fundamental principles for each category are presented, but some typical applications and softwares using MC method are also particularly introduced. On the basis of these applications, it is believed that the MC method plays an indispensable role in the development of diffuse optical imaging, especially in vivo, no-invasive imaging in recent years.