An assessment of the validity of studies is an essential component of most evidence syntheses (systematic reviews) to understand the risk of bias (ROB) and applicability of the evidence. A formal validity assessment requires a structured and comprehensive approach, which can be implemented using an assessment tool, specifically developed for this purpose. Many different tools are available, marking it difficult for researchers to choose the best tool for their evidence synthesis. We have established the LATITUDES Network to assist researchers in identifying the most appropriate tool to use in their evidence synthesis and to support researchers using these tools. The LATITUDES website (www.latitudes-network.org) includes a searchable library of validity assessment tools designed for use in evidence syntheses, bringing tools together in one place and providing researchers with clear information on suitable tools, categorized by study design. The website also provides links to training on the process of validity assessment and a list of tools currently under development. To be included in the LATITUDES library, tools must meet the following criteria: be designed for use in evidence syntheses; assess multidimensional aspects of validity of individual studies or reviews; and be developed for use by the wider research community rather than for a single research group. We highlight 'key' tools, those that are considered to be the most robust and reliable tools based on prespecified criteria agreed in conjunction with our advisory board, an international group of experts in the area of evidence synthesis and ROB tools.