Amino acids function as the building blocks of proteins, as energy sources, and as precursors for a number of different biological molecules. The chemical properties of amino acid side chains largely determine the structural and functional characteristics of proteins. Multiple metabolic pathways exist for the synthesis and catabolism of different amino acids. Mutational impairment of these pathways can cause a number of different inherited diseases that often cause significant neurological dysfunction. Analysis of amino acids is an effective diagnostic approach and many of these conditions are amenable to treatment.