Protein is one of the major components of the pig’s body, and dietary protein is the sole source of essential amino acids required for the protein synthesis for body maintenance, growth and reproduction. The protein synthesis is limited when there is a deficiency of any amino acid, and it is therefore important to ensure that feed meets the amino acid requirements of pigs at their various growth stages. This chapter reviews the principles and practical aspects of meeting the amino acid requirements of pigs, with information organized according to the growth stage and physiological status.