There are promising indicators of demand for reviews, including actions being taken to promote the use of reviews [6, 11, 19, 30, [32] [33] [34] [35] [36] , an awareness of sites providing access to review-level evidence [19] and an increasing number of groups generating summaries of reviews [33] . Despite this heightened activity, given the gaps, a greater investment is needed to provide an evidence base that can meet demand and determine how to apply existing good quality systematic reviews in different contexts [25] . Organizations involved in the conduct of systematic reviews should direct synthesis funding to areas lacking in review content, or should consider higher-level reviews of reviews (where appropriate), where large bodies of review evidence already exist. Examples of organizations that conduct systematic reviews and reviews of reviews include: The Cochrane Database of Systematic Reviews http://www.cochrane.org/reviews/ index.htm, The Campbell Collaboration http://www. campbellcollaboration.org, The Centre for Reviews and Dissemination (CRD) http://www.york.ac.uk/inst/crd/ index_databases.htm, Health Technology Assessment international (HTAi) http://www.htai.org/, Effective Public Health Practice Project (EPHPP) http://www.ephpp. ca, CDC Guide to Community Preventative Services http://www.thecommunityguide.org, Canadian Agency for Drugs and Technology in Health (CADTH) http:// www.cadth.ca/, Agency of Healthcare Research and Quality (AHRQ) http://www.ahrq.gov/ and the National Institute for Health and Clinical Excellence (NICE) http://www.nice.org.uk. Some groups have a prioritization process to identify and meet needs for systematic reviews; for example, the Agency of Healthcare Research and Quality has a topic prioritization group to determine the relative importance of their effectiveness reviews against a standard set of criteria to prioritize unmet needs [37] . The Cochrane Collaboration aims to increase the quantity and quality of public health systematic reviews specifically [32] , with the new Health Promotion and Public Health Review Group announced in 2006 to prioritize and produce public health relevant reviews [38] . Authors have suggested that a global registry of anticipated public health studies could help to fill the gaps by making it easier to identify relevant, but potentially unpublished, primary studies available for review [6] .