Protein phosphorylation plays key roles in many physiological processes and is often deregulated in pathological conditions. Our current understanding of how protein kinases and phosphatases orchestrate the phosphorylation changes that control cellular functions has made these enzymes potential drug targets for the treatment of many diseases. The success of the tyrosine kinase inhibitor Gleevec in the treatment of some cancers has further invigorated the development of kinase inhibitors as anti-cancer drugs. A large number of these compounds are currently undergoing clinical trials and there is much expectation on the therapeutic potential of these molecules, as more specific and less toxic drugs than currently used generic chemotherapeutic agents. In this manuscript, we review the current status of more than 30 protein kinase inhibitors with proven or potential therapeutic value for cancer treatment. These include inhibitors of receptor and cytosolic tyrosine kinases as well as compounds that target different families of serine/threonine kinases involved in signalling and cell cycle regulation. We also briefly touch on the prospects of phosphatase inhibitors. The combination of kinase inhibitors to target different components of signalling pathways that are found deregulated in tumours is also emerging as an interesting approach for cancer therapy.