The evaluation of biological methods of measuring the protein values of feeds will be considered under the following headings: 1. The basic facts in protein digestion and metabolism pertinent to the problem; 2. The essentials of the ideal solution; 3. Permissible modifications of these essentials consistent with reasonably satisfactory results; and 4. The limitations of some so-called practical methods. Since experimental methods in protein digestion and protein metabolism, as they relate to the nutritive evaluation of dietary protein, are concerned with the disposal of nitrogen in the body, the terms “nitrogen” and “prorein” will be used interchangeably in the following discussion.