Systematic reviews involve systematically searching for all available evidence, appraising the quality of the included studies, and synthesising the evidence into a usable form. They contribute to the pool of best available evidence, translating research into practice, and are powerful tools for clinicians, policymakers, and patients.1

To be useful for decision making, systematic reviews need to include high-quality evidence. However, there are systemic failings with the publishing, reporting, and interpretation of much of the evidence base, which undermine the findings of systematic reviews.2 3 In some cases, the evidence remains hidden from view or, when published, important outcomes are selectively reported, further hindering interpretation of reviews.4 Also, multiple interventions often have not been compared head to head, requiring more complex and indirect methods of evaluation. Finally, best practice guidance needs to be adaptable to real-world practice scenarios, which often requires combined information from multiple sources of evidence.5

To plug the evidence-to-practice translational gap, review methods are evolving beyond conventional ‘what works’ systematic reviews. Broader forms of evidence synthesis have emerged, such as network meta-analysis, scoping reviews, realist reviews, umbrella and meta-narrative reviews, meta-synthesis, and several others.6 7 Cochrane groups have expanded, with the creation of specific methods groups to reflect the growing number of review types.

The recognition that complex clinical and policy questions require more advanced methods of evidence synthesis to answer them has led to the term ‘complex review’, which is used to cover a wide range of evolving methods. However, there is some uncertainty as to what a complex systematic review is.