The importance of hydrolyzed food proteins in food science is on constant growth. Proteins are generally hydrolyzed to enhance their functional properties (e.g., food texture), to increase their intestinal absorption, or to release biologically active peptides. The property of the hydrolyzed proteins is dependent on several factors, including the physicochemical characteristics, free amine, nitrogen content, and the degree of hydrolysis. The quantification of proteins, peptides, and the determination of the free amino nitrogen content are important to assess the hydrolytic power of a protease and batch-to-batch reproducibility of protein hydrolysates. Procedures for the analysis of amine in proteins and mixtures of peptides, such as protein hydrolysates, have been developed. The measurement of free amine based on the derivatization with ortho-phthaldialdehyde (OPA) is advantageous as the detection can be done based on absorbance or fluorescence. This chapter discusses the basic principles, methodology, and calculations involved in amine quantitation in native and hydrolyzed food proteins using the OPA method.