Overviews of systematic reviews: great promise, greater challenge.

The proliferation of systematic reviews and escalating demand from policy makers has driven a newer form of evidence synthesis-overviews of systematic reviews. Systematic Reviews are publishing a special thematic series on overviews and are encouraging submissions on the development and evaluation of methods for this review type. The authors' of this editorial introduce the series by considering challenges that arise when conducting an overview and what methods guidance is available. They emphasise the importance of evaluating overview methods to understand the trade-offs of using different approaches and propose that a more systematic and coordinated approach to methods development would be beneficial. Finally, they consider the potential for overviews to drive improvements in the conduct and reporting of systematic reviews.