Pyridoxal phosphate (PLP)-dependent enzymes catalyze broad range of reactions. They are involved in biotransformation of amino acids and their derivatives in bacteria, fungi, plants and animals. Recently, due to development of crystallographic technology, three-dimensional structures of some PLP-enzymes are being solved. A comparison of tertiary structure of these enzymes has shown that most of them can be divided into three classes of homologous proteins. This review contains short characteristics of PLP-dependent enzymes, regarding to their substrates and the reaction types catalyzed. Because of wide distribution and involvement in numerous important cellular processes, these enzymes are considered as candidates for drug targets. Better understanding of structures and functions of this important group of enzymes is essential for designing their inhibitors and for synthesis of improved protein-based catalysts.