In 2011, more than 240,000 men are projected to be diagnosed with prostate cancer, and 33,000 are projected to die from the disease in the United States. In the United States, most cases of prostate cancer are detected via prostate-specific antigen (PSA) screening. The cancer is usually localized, and most tumors have low histological grades and low Gleason scores. Indeed, more than half of prostate cancers detected by PSA screening are expected to be early-stage, low-risk tumors. Such cancers are an infrequent cause of death, and those affected are more likely to die of unrelated causes. A number of immediate active treatment options are available for localized prostate cancer. Most commonly, radical prostatectomy (RP) or radiation therapy (RT), with or without androgen deprivation therapy (ADT), are offered with curative intent. However, the clinical benefit of immediate therapy with curative intent has not yet been demonstrated for localized prostate cancer in a PSA-screened population. It is likely that a large number of men are receiving active treatment with curative intent without much likelihood of obtaining any clinical benefit due to the slow progression of many prostate tumors. Both surgical and radiation treatments result in significant short- and long-term adverse events, including impotence, urinary dysfunction, and other complications. Thus, determination of the appropriate management strategy for early-stage, low-risk prostate cancer is an important public health concern. Active surveillance (AS) and watchful waiting (WW) are two observational followup strategies that forgo immediate therapy in patients with prostate cancer, with the goal of minimizing the morbidities and costs of immediate active treatment for men who may never develop cancer-related symptoms or who are interested in palliative treatments only. AS is curative in intent, and WW is palliative. AS is appropriate in men with disease believed to be indolent and therefore may not require therapy. Because prediction tools are imperfect, these men are monitored closely and treated with curative intent at signs of progression or patient choice. In this way, the considerable adverse effects of treatment are at best avoided, and at least deferred. This approach is to be distinguished from men for whom treatment is deemed inappropriate because of comorbidity; for these men, WW is generally considered, as it offers the option of palliative therapy upon symptomatic disease progression. AS often entails a multifactorial followup of patients-monitoring of PSA values, digital rectal examinations (DRE), prostate imaging, and periodic prostate biopsies-while WW is commonly a relatively passive strategy-with interventions triggered by symptoms. However, there is a continuum of aggressiveness of followup for both AS and WW, as practiced in the community. The objective of this report is to summarize the existing literature regarding the role of AS in the management of early-stage, low-risk prostate cancer. Both the report and the corresponding NIH State-of-the-Science conference are a part of the NIH Consensus Development Program, the purpose of which is to evaluate the scientific evidence on a particular topic and develop a consensus statement that advances research in that area. Key Questions provided to the EPC for systematic review include: 1. How have the patient population and the natural history of prostate cancer diagnosed in the United States changed in the last 30 years? 2. How are active surveillance and other observational management strategies defined? 3. What factors affect the offer of, acceptance, and adherence to active surveillance? 4. What are the comparative short- and long-term outcomes of active surveillance versus immediate treatment with curative intent for localized prostate cancer? 5. What are the research needs regarding active surveillance (or watchful waiting) in localized prostate cancer?