There are many ways to analyse and quantify the proliferating activity of a tumor. The different technics are more and more specific of each phase of the cell cycle. This paper firstly describes the basis of each of these technics (mitosis count, DNA staining with various compounds fluorescent or not, incorporation of specific molecules during the phase of DNA synthesis, detection of proliferation antigens by immunological methods). Secondly, the interest of the proliferating fraction knowledge of a tumor is discussed, for various pathologies and according to the technic used.