Members of the protein tyrosine phosphatase (Ptp) family dephosphorylate target proteins and counter the activities of protein tyrosine kinases that are involved in cellular phosphorylation and signalling. As such, certain PTPs might be tumour suppressors. Indeed, PTPs play an important part in the inhibition or control of growth, but accumulating evidence indicates that some PTPs may exert oncogenic functions. Recent large-scale genetic analyses of various human tumours have highlighted the relevance of PTPs either as putative tumour suppressors or as candidate oncoproteins. Progress in understanding the regulation and function of PTPs has provided insights into which PTPs might be potential therapeutic targets in human cancer.