Amino acids (AAs) are defined as organic compounds containing both amino and acid groups (1). Proteinogenic AAs are AAs that serve as substrates for protein synthesis in animal cells, and occur naturally as L-AAs except for glycine. Free AAs (those AAs that are not constituents of peptides or polymers) in physiologic fluid exist primarily in an L-isoform, but some of them may be present in a D-isoform. Based on his findings in 1912 that adult dogs exhibited a negative nitrogen balance when fed a tryptophan-free diet but maintained a positive nitrogen balance when fed a proline-free diet, Abderhalden (2) classified AAs as nutritionally essential (EAAs) or nonessential (NEAAs). Beginning in 1924, WC Rose and coworkers (3) published a series of landmark papers on AA nutrition and metabolism in rats and humans that further defined AAs as EAAs or NEAAs based on nitrogen balance or growth. We published an essay on NEAAs in 2017 (4), and EAAs are the focus of the present article.