  We present the Herschel Extragalactic Legacy Project (HELP). This project
collates, curates, homogenises, and creates derived data products for most of
the premium multi-wavelength 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 around the homogenisation of calibration, meta data 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 modelling the
spectral energy distributions.
