Jarvis, M.J.Shirley, RDuncan, KCampos Varillas, M.C.Hurley, P.D.Malek, KRoehlly, YOliver, S.J.2021-09-282021-09-282021Shirley, R., Duncan, K., Campos Varillas, M. C., Hurley, P. D., Małek, K., Roehlly, Y., . . . Oliver, S. J. (2021). HELP: The herschel extragalactic legacy project. Monthly Notices of the Royal Astronomical Society, 507(1), 129-155. doi:10.1093/mnras/stab15260035871110.1093/mnras/stab1526http://hdl.handle.net/10566/6831The research leading to these results has received funding from the European Union Seventh Framework Programme FP7/2007-2013/under grant agreement No. 607254 (The Herschel Extragalactic Legacy Project). This publication reflects only the authors' view and the European Union is not responsible for any use that may be made of the information contained therein. SO acknowledges support from the Science and Technology Facilities Council (grant number ST/L000652/1) KM has been supported by the Polish National Science Centre (UMO-2018/30/E/ST9/00082).We present the Herschel Extragalactic Legacy Project (HELP). This project collates, curates, homogenizes and creates derived data products for most of the premium multiwavelength extragalactic data sets. The sky boundaries for the first data release cover 1270 deg2 defined by the Herschel SPIRE extragalactic survey fields; notably the Herschel Multi-tiered Extragalactic Survey (HerMES) and the Herschel Atlas survey (H-ATLAS). Here, we describe the motivation and principal elements in the design of the project. Guiding principles are transparent or 'open' methodologies with care for reproducibility and identification of provenance. A key element of the design focuses on the homogenization of calibration, metadata, and the provision of information required to define the selection of the data for statistical analysis. We apply probabilistic methods that extract information directly from the images at long wavelengths, exploiting the prior information available at shorter wavelengths and providing full posterior distributions rather than maximum-likelihood estimates and associated uncertainties as in traditional catalogues. With this project definition paper, we provide full access to the first data release of HELP; Data Release 1 (DR1), including a monolithic map of the largest SPIRE extragalactic field at 385 deg2 and 18 million measurements of PACS and SPIRE fluxes. We also provide tools to access and analyse the full HELP database. This new data set includes far-infrared photometry, photometric redshifts, and derived physical properties estimated from modeling the spectral energy distributions over the full HELP sky. All the software and data presented are publicly available. © 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.enAstronomical databasesCataloguesGalaxiesEvolutionInfrared: galaxiesGalaxies: evolutionSubmillimetre: galaxiesSurveysTechniques: photometricHerschel Extragalactic Legacy Project (HELP)HELPHerschel Multi-tiered Extragalactic SurveyHerMESHerschel Atlas surveyH-ATLASHomogenization of calibrationMetadataArticle