The PAU Survey is an international collaboration between Spain, the Netherlands, Switzerland, Germany and the UK to collect a fluxed limited accurate redshift and spectral energy distribution information for millions of galaxies (and stars) to a depth and area never explored before. This is done without the need of target selection which provides a valuable tool to understand sample selection and completeness. To do this, a spanish group had built PAUcam, a large imager (1 degree square FoV, with18 CCD of 4094x2048 pixels each) capable of doing imaging with 40 narrow-band filters (~130A wide in steps of 100A) and 6 wide-band filtres (u,g,r,i,z,Y), which results in high resolution spectra energy distributions for all galaxies in a given field of view. A number of large surveys (e.g. KiDS, DES, SDSS, LSST, DESI) seeks to map the 3D structure of the Universe to understand the nature of dark matter and dark energy. Surveying large contiguous areas while simultaneously achieving a high number density of galaxies with sub-percent redshift accuracy is possible thanks to PAUCam. This exquisite wavelength sampling results in photometric redshifts with a precision that approaches that of spectroscopic measurements, while being able to cover large areas of sky.