As a color consistence model derived from human visual system research, Retinex has been widely used in processing uneven illumination and color shift images. This paper first introduces the fundamental principle and subsequent development of Retinex theory. Then according to the current research status, the Retinex model is divided into four types: Path-based model, PDE (Partial differential equations) model, variational model and center-surround model, and each type of model is reviewed. Finally, the paper summarizes the advantages and disadvantages of the four Retinex models, and introduces the typical application of Retinex in image enhancement, as well as the future development direction.