The present invention relates to novel phosphonoamidate prodrugs. The invention also relates to the use of these novel phosphonate-modified nucleosides to treat or prevent viral infections and their use to manufacture a medicine to treat or prevent viral infections, particularly infections with viruses such as the herpes simplex virus 1, herpes simplex virus 2, human cytomegalovirus, varicella zoster virus, vaccinia virus and adenovirus.