The natural history of prostate cancer is often of long duration, and the disease is incompletely understood. Whether all men with prostate cancer require immediate treatment or whether men with tumors of low malignant potential are being overtreated with potentially harmful therapies is a subject of much debate. Results from a randomized trial that compared watchful waiting and active therapy showed all-cause and disease-specific survival advantages with radical therapy, but the study group was mixed in terms of disease risk; the optimum treatment strategy for men with low risk features remains unclear. Multiple centers are gaining experience with active surveillance and delayed intervention with curative intent for men with prostate tumors of potentially low clinical risk. This Review describes the background studies behind the rationale for active surveillance, thoughts on selection criteria for candidates and some early reported outcomes for active surveillance cohorts. The psychosocial impact of active surveillance on patients is discussed as well as contemporary methods for disease monitoring.