The invention provides for compositions comprising phosphorous containing tricyclic compounds, including phthalazin-1(2H)-one derivatives. The compounds are potent inhibitors of the enzyme poly(ADP-ribose)polymerase (PARP), particularly PARP-1 and potentially PARP-2. The also show good cellular activity in inhibiting poly(ADP-ribose) oligomer formation. The compounds may be useful as mono-therapy or in combination with other therapeutic agents in the treatment conditions where PARP is implicated, such as cancer, inflammatory diseases and ischemic conditions. Thus, also provided are methods for the treatment of a condition where PARP is implicated comprising administering to an effective amount of a compound of the invention to an individual in need thereof.