Corremans, M.Munn, Z.Peters, S.Jonckheer, P.Parisod, H.Lenaerts, G.Karam, M.Durieux, N.Leclercq, A.L.Boers, A.Vandevijvere, H.2025-10-162025-10-162025JBI Evidence Implementation, 2025; 23(4):518-5242691-33212691-3321https://hdl.handle.net/2440/147816Original ResearchThe Medical Research Council proposed a framework to develop and implement complex interventions in practice. How to adopt these interventions is described in recommendations of evidence-based clinical practice guidelines. Many factors may influence the complexity of a guideline. The aim of this paper is to describe the determinants of complexity in the development and implementation of an evidence-based clinical practice guideline. A working group with 16 participants was established, consisting of a debate team and a Delphi panel. The debate team discussed online to define the key elements of the MRC's definition of a complex intervention to see whether these elements are applicable to guidelines. These elements were presented to the Delphi panel to assess their relevance. After the first round, consensus was reached on eight elements, with the inter-rater reliability varying from 0.83 to 1.00. After the second Delphi round, consensus was reached on two more elements. The consensus stated that these ten elements all define an aspect of the complexity in guidelines. There was no agreement regarding the exclusion of a specific element. Developers and end-users consider that the complexity of a guideline and its implementation is affected when the number of components, settings, targeted behaviors, and stakeholders increase; when a gap exists between the guideline and the reality of clinical practice; or when differences in education are evident between end-users. Moreover, the level of collaboration required of the different end-users, the scope of change, the level of evidence in the guideline, and the workload for end-users also determine complexity.en©2025 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the University of Adelaide, JBI. This is an open access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.complexity; determinants; guidelines; evidence-based practice; implementation scienceJournal article10.1097/XEB.0000000000000499732684Munn, Z. [0000-0002-7091-5842]