Dairy production is associated with significant losses of nitrogen to the environment, contributing to the degradation of air and water systems. These losses must be decreased to make dairy farming sustainable, and a primary means of doing this is to increase the conversion of feed protein into milk protein. This chapter considers the two nitrogen-utilising systems in the cow: the rumen microbiota and ruminant tissues. The more efficient both the rumen and tissue systems are, the lower the urinary nitrogen excretion. We review approaches to estimating rumen microbial protein synthesis, the protein and energy requirements of dairy cows and the use of milk urea nitrogen (MUN) to assess the nitrogen efficiency of dairy cows. We then consider the development of nutritional systems which account for rumen microbial synthesis, rumen-degradable and undegradable feed protein and endogenous protein supplies of amino acids based on utilisation of feed inputs.