The acyclic nucleoside phosphonate analogues (HPMPA, HPMPC, PMEA, FPMPA) show great promise for the treatment of infections with such important human pathogens as adeno, pox (vaccinia) and hepadna (hepatitis B) viruses (HPMPA), herpes (herpes simplex, varicella-zoster, cytomegalo, Epstein-Barr) viruses (HPMPC), and retro (human immunodeficiency) viruses (PMEA, FPMPA). All these compounds seem to be targeted at the viral DNA polymerase, with which they interact, as either competitive inhibitors or alternative substrates (or chain terminators), following their intracellular phosphorylation to the diphosphoryl derivatives. Of particular interest is the prolonged anti-viral action, lasting for several days or even weeks, that has been noted both in vitro and in vivo after a single administration of the acyclic nucleoside phosphonates.