The Herschel Multi-tiered Extragalactic Survey (HerMES), the largest Key Project of the Herschel Space Observatory (Oliver et al. 2011), was designed to map a set of nested fields totaling 100 square degrees and ranging in size from 0.01 to 20 square degrees, using Herschel-SPIRE (at 250, 350 and 500 microns), and Herschel-PACS (at 100 and 160 microns), with an additional wider component of a few 100 square degrees with SPIRE alone (HELMS survey). These bands capture the reradiated optical and ultra-violet radiation from star formation that has been absorbed by dust, and are critical for forming a complete multi-wavelength understanding of galaxy formation and evolution. The survey will detect 50–100 thousand galaxies at 5 sigma in some of the best studied fields in the sky. Making maximum use of the full spectrum of ancillary data, from radio to X-ray wavelengths, it will be possible to: facilitate redshift determination; rapidly identify unusual objects; and understand the relationships between thermal emission from dust and other processes. Scientific questions HerMES will be used to answer include: the total infrared emission of galaxies; the evolution of the luminosity function; the clustering properties of dusty galaxies; and the properties of populations of galaxies which lie below the confusion limit through lensing and statistical techniques. This talk will review the HerMES survey and present the first results of the follow-up programme of HerMES sources with OSIRIS at GTC, including long-slit spectroscopy of SPIRE-selected sources (lensed and unlensed) and deep broad-band imaging.