Abstract Background Measuring dietary intake is difficult, and strategies that enable researchers to select the most appropriate dietary assessment tools are needed. The aim of this work was to improve the quality of dietary data collected in epidemiological studies. Therefore, the DIETary Assessment Tools NETwork (DIET@NET) partnership, a network of scientific experts, has created the Nutritools website. Methods Development of the Nutritools website was divided into three strands: creation of best practice guidelines, developed with the Delphi technique to obtain expert views (the guidelines enable researchers to choose the most appropriate dietary assessment tool for their work); creation of an interactive dietary assessment tool e-library, with eligible dietary assessment tools being identified through a systematic review of reviews that searched seven databases; and creation of an online interface between food tables and dietary assessment tools—namely, the Food Questionnaire Creator (FQC). The work was guided by the DIET@NET partnership. Findings Interactive dietary assessment guidelines were generated with feedback from 57 international experts. 43 guidelines and a summary of the strengths and weaknesses of the dietary assessment methods were included. The dietary assessment tool e-library included data for 62 UK validated tools, which were obtained from 43 systematic reviews identified. The tool library will provide in-depth information about the tools, validation study characteristics, and results. This information is also provided visually through bubble and summary plots, allowing easier comparison between the dietary assessment tools. The FQC was based on the principles of common food frequency questionnaires and allows users to create and develop new online food questionnaires. Users can map their online questionnaires to the latest UK food database (McCance and Widdowson's Composition of Foods, 7th edn). The FQC will also host a number of validated dietary assessment tools that have been adapted for online use. Interpretation The DIET@NET partnership has created a unique dietary assessment reference website (live from November, 2017) using expert guidance and systematic review. The website allows visual comparison of dietary assessment tools and hosts validated, interactive tools. The best practice guidelines assist researchers in selecting the most appropriate tool for their study. Researchers can access validated dietary assessment tools through the e-library and create their own using the FQC. Funding UK Medical Research Council (grant no MR/L02019X/1).