To quantify the microbial protein synthesis in ruminant animals is of great importance to assess the nutritional quality of diets. The determination of microbial synthesis is achieved through the use of microbial markers, which are classified: internal (By-products of rumen microorganisms) and external (products administered to animals that are incorporated by microorganisms). Despite the great effort by researchers in recent years, still do not have the ideal marker, and most of these advantages and disadvantages. So the goal of this review is to detail each of the techniques for estimating microbial protein synthesis in the rumen, and analyze and discuss the results obtained using each technique.