Renalase is a recently discovered secretory enzyme responsible for selective degradation of blood catecholamines. The review summarizes literature data on expression of this enzyme and on its structure and functions. Special attention is paid to unsolved and questionable problems including: 1) prediction of the presence of FAD in the protein structure based on amino acid sequence similarity of renalase with known FAD-dependent enzymes; 2) identity of plasma and urinary renalase; 3) mechanism underlying conversion of inactive renalase into the active form.