This chapter discusses the principles and practical aspects of meeting amino acid requirements and the use of functional amino acids in pig nutrition. The authors begin by exploring protein and amino acid metabolism, then the principles of amino acid needs, including maintenance and growth. They next describe the estimation of amino acid requirements, covering empirical and mechanistic approaches, and the availability and use of supplemental amino acids. Following this, the authors outline the concept and application of functional amino acids, such as arginine, methionine, tryptophan, phenylalanine, and glutamine. Finally they discuss the practical aspects of feed formulation meeting amino acid needs.