In December 1992, a meeting was held with Elsevier which led to the introduction of RCT as an indexing term into Embase in 1993  and a commitment by Elsevier to improve the indexing of clinical trials . This was followed in December 1993 by a conference hosted by the US National Library of Medicine (NLM) which led to agreement to 're-tag’ RCTs in MEDLINE (the MEDLINE re-tagging project). This project resulted in more than 125,000 reports of randomized and quasi-randomized trials, not already indexed as such in MEDLINE, being identified, re-tagged in MEDLINE and included in the Cochrane Central Register of Controlled Trials (CENTRAL).
In 1995, a new indexing term for quasi-randomized trials (Controlled Clinical Trial) was introduced into MeSH . The Cochrane Central Register of Controlled Trials (CENTRAL) was launched in 1996 (under its original name of The Cochrane Controlled Trials Register), as part of the newly published Cochrane Library. CENTRAL was referred to in its early days as 'likely to be the best single source of published trials for inclusion in systematic reviews and meta-analyses’ . In 1996, Elsevier agreed that reports of trials identified from Embase could be included in CENTRAL (the Embase project). CENTRAL developed rapidly over the next few years . The contributions of the MEDLINE re-tagging project [6, 8] and the Embase project  now form the basis of CENTRAL, which is the single largest source of reports of trials, with 700,000 records drawn from MEDLINE, Embase, Cochrane groups and other sources .
Considerable progress has been made in searching across a range of areas important to the systematic review process. Some examples include the identification of information on adverse effects of interventions [10–14]; diagnostic test studies [15–17]; economic evaluation [18–21]; studies of prognosis [22–24] and causation [24–27]; non-randomized studies related to interventions  and qualitative studies [29–33]. Progress has also been made in the identification of systematic reviews, as sources of potential studies [34–37].
With respect to RCTs, filters aimed at identifying all RCTs in MEDLINE, irrespective of other study characteristics, began to be developed by members of The Cochrane Collaboration in the early 1990s . They were revised using objective methods of search strategy design (textual analysis) in 2006  and subsequently kept up to date in the 'Searching for Studies’ chapter of The Cochrane Handbook for Systematic Reviews of Interventions.
The proliferation of search filters across a range of methodological areas, and their ease of access through incorporation in services such as Ovid and PubMed, has led to the development of the InterTASC Information Specialists’ Sub-Group (ISSG) Search Filter Resource. This offers critical appraisals and summaries of search filters together with references to comparative testing data [40, 41]. Given the growing interest in identifying information beyond particular types of study, such as age groups, geographic areas and ethnic groups, the ISSG Search Filter Resource has recently been expanded to incorporate these topics. Examination of the conduct and reporting of searches for Cochrane Reviews [42, 43] has led to structured approaches to peer review of search strategies (for example, the Peer Review of Electronic Search Strategies (PRESS) checklist) [44–47].
In addition to the advances in the identification of studies from bibliographic databases outlined above, methodological work has been undertaken on the value of searching the 'grey literature’ which has been defined as 'information produced and distributed at all levels by government, academics, business and industry in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body’ [48, 49]. Related research has assessed the value of handsearching for trials [50–52] and examined the characteristics associated with full publication of meeting abstracts . Alternative search techniques such as 'pearl-growing’/'snowballing’ from known key references and checking reference lists have also been explored [54, 55].
Keeping up-to-date in methodological advances has been facilitated by the advent and development of the Cochrane Methodology Register, published in The
Cochrane Library. Updating of this resource is currently on hold pending decisions regarding its future within The Cochrane Collaboration. The recently-launched Summarized Research in Information Retrieval for Health Technology Assessment (SuRe Info) provides research-based information regarding the latest developments in the information retrieval aspects of producing systematic reviews and health technology assessments . It provides critical appraisals and summaries of current methods papers and general overviews of the state of the evidence across a range of topics relevant to information retrieval for systematic reviews.
The guidance in Chapter 6 of The Cochrane Handbook for Systematic Reviews of Interventions entitled 'Searching for Studies’ started life in 1994 as an internal Cochrane document produced by Kay Dickersin and Carol Lefebvre entitled 'Establishing and Maintaining Registers of RCTs’. The document provided limited guidance with respect to the conduct and reporting of searches. This subsequently became incorporated into the Handbook and now provides detailed guidance for authors of Cochrane Reviews and Cochrane Review Group staff, including TSCs, Managing Editors, Co-ordinating Editors and Editors . It is also used by other evidence synthesis organizations and provided a model for the Campbell Collaboration’s Guide to Information Retrieval for Campbell Systematic Reviews . The Handbook is revised and updated in consultation with the information retrieval community of The Cochrane Collaboration, that is, the Cochrane Information Retrieval Methods Group and TSCs. Standard Training Materials have been produced and have been updated in the light of the standards recently introduced under the Cochrane Methodological Expectations of Cochrane Intervention Reviews programme (MECIR) .
In 2013, almost all Cochrane Review Groups and some Cochrane Centres and Fields have a dedicated TSC – usually a qualified librarian/information specialist with experience of searching the medical literature. These TSCs carry out a vital role in study identification within their respective groups, although the nature of their contributions varies considerably according to resources and other factors. There is also far greater awareness amongst medical librarians and other information specialists regarding the role of systematic reviews and how to search for studies for inclusion in systematic reviews.