Phosphoramidate derivatives of nucleotides and their use in the treatment of cancer are described. The base moieties of, for example, each of deoxyuridine, cytarabine, gemcitabine and citidine may be substituted at the 5-position. The phosphoramidate moiety has attached to the P atom an aryl-O moiety and an .alpha.-amino acid moiety. The a-amino acid moiety may correspond to or be derived from either a naturally occurring or a non-naturally occurring amino acid.