This document discusses the 20 standard amino acids that make up proteins. It covers their common structures, including the alpha carbon and functional groups. The amino acids are classified based on properties of their functional groups as nonpolar, aromatic, polar uncharged, negatively charged, and positively charged. Their one-letter abbreviations are also introduced. Spectroscopic properties of phenylalanine, tyrosine, and tryptophan are discussed. Disulfide bonds between cysteine residues are mentioned.