Abstract: The optical information will experience the scattering phenomena, when it propagates in the scattering media, which will change the intensity and polarization information of the propagating light. And the depolarization property and the transmission property of the medium can be characterized indirectly, which can be used to classify and recognize the media. In theory, the Mueller Matrix (MM) can describe all polarization properties of the media, which plays a vital role in analyzing the depolarization properties of the medium, but its parameters are too complicated. However, the index of polarization purities (IPPs) obtained from the Mueller matrix can also describe the polarization properties of the media directly. IPPs are composed of P_1, P_2, P_3, which represent the weight differences of four non-depolarization pure systems decomposed from the depolarization system equivalently. The P_1, P_2, P_3 can form a three-dimensional space called purity space, in which different points represent corresponding depolarization systems. And they can be used for recognizing different depolarization systems. Compared to standard polarization indexes, the IPPs can express the more dimensional information of the scatter media, which has obtained many important research achievements in many aspects such as biomedical and target detection. We mainly introduce the IPPs theory, and meanwhile, review and discuss its great role in the analysis of depolarization of the different dispersion system, biological tissue imaging, medical monitoring and target recognition.