| Active surveillance (AS) is broadly described as a management option for men with low-risk prostate cancer, but semantic heterogeneity exists in both the literature and in guidelines. To address this issue, a panel of leading prostate cancer specialists in the field of AS participated in a consensus-forming project using a modified Delphi method to reach international consensus on definitions of terms related to this management option. An iterative three-round sequence of online questionnaires designed to address 61 individual items was completed by each panel member. Consensus was considered to be reached if ≥70% of the experts agreed on a definition. To facilitate a common understanding among all experts involved and resolve potential ambiguities, a face-to-face consensus meeting was held between Delphi survey rounds two and three. Convenience sampling was used to construct the panel of experts. In total, 12 experts from Australia, France, Finland, Italy, the Netherlands, Japan, the UK, Canada and the USA participated. By the end of the Delphi process, formal consensus was achieved for 100% (n = 61) of the terms and a glossary was then developed. Agreement between international experts has been reached on relevant terms and subsequent definitions regarding AS for patients with localized prostate cancer. This standard terminology could support multidisciplinary communication, reduce the extent of variations in clinical practice and optimize clinical decision making.312 | MAY 2017 | VOLUME 14 www.nature.com/nrurol CONSENSUS STATEMENT © 2 0 1 7 M a c m i l l a n P u b l i s h e r s L i m i t e d , p a r t o f S p r i n g e r N a t u r e . A l l r i g h t s r e s e r v e d .degree of monitoring than those on AS might receive, and in which palliative treatment is generally instituted if metastases or local symptoms develop. Interpreting and comparing research results is difficult owing to the nonstandardized use of the terms AS and WW and their intended and often mixed treatment objectives (curative or palliative) 5 . This semantic heterogeneity is also reflected in AS guidelines 6 . In these guidelines, AS is primarily recommended for patients with low-risk tumours; however, these guidelines contain various definitions of low-risk prostate cancer, as specified by different combinations of clinical criteria including clinical and pathological characteristics (such as tumour stage, serum PSA levels, biopsy Gleason score, tumour volume and serum PSA density). Furthermore, definitions of disease reclassification and progression differ among published guidelines and multiple criteria for initiation of curative treatment are proposed 6 .Problems resulting from the use of ambiguous language include hindered clinical decision making, particularly in multidisciplinary collaborations, and limited opportunities for research 7 . Moreover, such ambiguity has raised a barrier that hampers exchange of knowledge within and between fundamental domains of research and research groups 8 . An urgent need exists for uniform...