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Amino acids are the most important components of black tea and are associated with the physiological and biochemical changes that occur during tea processing. Herein, we review the enzymatic and non-enzymatic reactions of the main proteinaceous and non-proteinaceous amino acids that occur during the manufacturing process of black tea from the postharvest processing steps of tea leaves to the final product. Enzymatic reactions are responsible for the amino acid variation in the withering, rolling, and fermentation stages. The significant increase in the total amino acid content during the withering stage is mainly derived from proteolysis. Furthermore, the transformation reactions in the rolling and fermentation stages decrease the amino acid levels slightly, whereas the non-enzymatic reactions, such as the Maillard and Strecker reactions, contribute to the significant decrease in the amino acid content of the drying stage. This study provides deep insights into the mechanisms underlying amino acid variation during black tea processing. [ABSTRACT FROM AUTHOR]