The article describes the basics of the synthesis of microbial protein in the rumen and the factors which influence this process. Literature data and our own research found that the following factors influence synthesis of microbial protein in the rumen of ruminants: dry matter intake, the role of nitrogen compounds, energy supply from fermentative (enzymic) processes in the rumen, correlation of rough forage to concentrates in feed, environment of the rumen (pH), the rate of passage of forage masses and the duration of presence of forage masses in the rumen, and the role of the vitamins and minerals. All these factors have a direct effect on the synthesis of microbial protein in the rumen. Therefore, in this article each factor has separate discussion.