This chapter sketches those significant developments that have added to the present nutritional concepts regarding proteins, and considers more extensively the current active field of investigation into the effects of various processes upon protein quality. Several basic procedures have been employed in studies of the nutritive value of protein, each involving, in some way, a measurement of the response of test animals to the protein under investigation. Those employed most frequently are determinations of biological value by nitrogen balance experiments, protein efficiency measurements relating weight gain to protein consumed, and techniques for measuring the ability of animals to form or regenerate specific body proteins from dietary protein. Digestibility and amino acid composition are the two principal properties involved in biological value measurements. The application of these methods by a number of investigators to hydrolyzates of proteins or protein-containing foods has resulted in the accumulation of considerable information regarding the amino acid composition of foods. Such an estimate of probable biological value has been shown to have considerable merit and has been discussed briefly. Factors contributing to the biological value of protein have been discussed and different treatments have been classified in such a manner as to permit generalizations regarding their effects. For this purpose, operations such as pickling and curing, dehydration, and storage have been considered. Selected illustrations have been tabulated, showing losses of nutrients in the preparation of foodstuffs. The chapter discusses the effects of heat on relatively pure protein, proteins in mixtures of other foodstuffs, proteins of soybean and cottonseed meals and related products, protein of meat, fish, and other animal products, and the relationship of the browning reaction to changes occurring in pooch Studies of the effects of storage are studied and few reports of changes in the quality of protein during storage have been mentioned.