The review summarizes the recent knowledge on the group of biologically active compounds called pterins. Biosynthesis of all known pterins starts from quanosine triphosphate and it is in vivo stimulated by cytokines, mainly by interferon gamma. Pterins can exist in three oxidation states, the reduced forms of which are relatively unstable. More details on the biological function are known only for some pterins. Tetrahydrobiopterin is an essential cofactor of aromatic amino acid monooxygenases and its role in the synthesis of nitric oxide has been recently studied. Neopterin is a sensitive marker of activated cell-mediated immunity and may be involved in processes mediated by oxygen radical. Molybdopterin is a component of molybdenum cofactors that participate in redox reactions. Oncopterin was found in urine of patients with malignant lymphoma and is expected to be a promising biochemical marker of some cancers. The HPLC with fluorescence or electrochemical detection are the predominating analytical methods for the determination of pterins.