Amino acids are first and foremost known as building blocks for the synthesis of proteins and peptides in all biological organisms, from archaea and bacteria up to mammals including humans. The sequences of amino acids in proteins, which are determined by the sequences of nucleotides within DNA, according to the (nearly) universal genetic code for all living organisms, together with the posttranslational modifications of proteins, determine their chemical structure and therefore their functions in the different biological organisms. However, numerous amino acids found in the living world are not incorporated into proteins but play metabolic and physiological roles on their own. Amino acids are used by living organisms as precursors of a myriad of compounds with various and central metabolic and physiological functions. In this introductory chapter, we briefly recapitulate the main characteristics and metabolic functions of amino acids in living organisms. These functions include not only their role as precursors for peptide and protein synthesis, but also as precursors for the synthesis of DNA and RNAs, as substrates involved in ATP production, and as precursors of bioactive compounds.