This chapter is meant to provide the provider a quick and up-to-date overview of proteins in general. The basic structure of proteins includes primary, secondary, and quaternary forms. The amino acids are grouped into essentials, nonessential, and conditionally essential amino acids. The digestion of proteins starts in the stomach and continues into the duodenum with the pancreatic component and ends in the small intestine. The absorption of amino acids and small peptides is facilitated by specific transporters in the small bowel. Disorders of digestion and absorption of protein are addressed in the latter part including lysinuric protein intolerance.