Prostate cancer is the only human cancer that is curable but which commonly does not need to be cured. Active surveillance is a new strategy that aims to individualise therapy by selecting only those men with significant cancers for curative therapy. Patients with favourable tumour characteristics are closely monitored using serum prostate specific antigen (PSA) concentrations and repeat prostate biopsies. The choice between radical treatment and continued observation is based on evidence of disease progression, defined in terms of the PSA doubling time, and "upgrading" at repeat biopsy. Active surveillance provides an excellent opportunity for studies to identify markers of prostate-cancer behaviour. Knowledge of prostate cancer biomarkers would have an immediate effect on clinical decision-making and would also identify targets for the development of novel therapeutic strategies. In the longer term, active surveillance may accelerate progress towards a new treatment paradigm for early prostate cancer based on the selective use of therapies designed, not to eradicate the disease, but to alter its natural history.