Cochrane methods--twenty years experience in developing systematic review methods.

This year, The Cochrane Collaboration reached its 20th anniversary. It has played a pivotal role in the scientific development of systematic reviewing and in the development of review methods to synthesize research evidence, primarily from randomized trials, to answer questions about the effects of healthcare interventions. We introduce a series of articles, which form this special issue describing the development of systematic review methods within The Cochrane Collaboration. We also discuss the impact of Cochrane Review methods, and acknowledge the breadth and depth of methods development within The Cochrane Collaboration as part of the wider context of evidence synthesis. We conclude by considering the future development of methods for Cochrane Reviews.