ForewordThe Pierre Auger Observatory has begun a major Upgrade of its already impressive capabilities, with an emphasis on improved mass composition determination using the surface detectors of the Observatory. Known as AugerPrime, the upgrade will include new 4 m 2 plastic scintillator detectors on top of all 1660 water-Cherenkov detectors, updated and more flexible surface detector electronics, a large array of buried muon detectors, and an extended duty cycle for operations of the fluorescence detectors.This Preliminary Design Report was produced by the Collaboration in April 2015 as an internal document and information for funding agencies. It outlines the scientific and technical case for AugerPrime 1 . We now release it to the public via the arXiv server. We invite you to review the large number of fundamental results already achieved by the Observatory and our plans for the future.The Pierre Auger Collaboration 1 As a result of continuing R&D, slight changes have been implemented in the baseline design since this Report was written. These changes will be documented in a forthcoming Technical Design Report. ix x Executive Summary Present Results from the Pierre Auger ObservatoryMeasurements of the Auger Observatory have dramatically advanced our understanding of ultra-high energy cosmic rays. The suppression of the flux around 5×10 19 eV is now confirmed without any doubt. Strong limits have been placed on the photon and neutrino components of the flux indicating that "top-down" source processes, such as the decay of superheavy particles, cannot account for a significant part of the observed particle flux. A largescale dipole anisotropy of ∼7% amplitude has been found for energies above 8×10 18 eV. In addition there is also an indication of the presence of a large scale anisotropy below the ankle. Particularly exciting is the observed behavior of the depth of shower maximum with energy, which changes in an unexpected, non-trivial way. Around 3×10 18 eV it shows a distinct change of slope with energy, and the shower-to-shower variance decreases. Interpreted with the leading LHC-tuned shower models, this implies a gradual shift to a heavier composition. A number of fundamentally different astrophysical model scenarios have been developed to describe this evolution. The high degree of isotropy observed in numerous tests of the small-scale angular distribution of UHECR above 4×10 19 eV is remarkable, challenging original expectations that assumed only a few cosmic ray sources with a light composition at the highest energies. Interestingly, the largest departures from isotropy are observed for cosmic rays with E > 5.8×10 19 eV in ∼20 • sky-windows. Due to a duty cycle of ∼15% of the fluorescence telescopes, the data on the depth of shower maximum extend only up to the flux suppression region, i.e. 4×10 19 eV. Obtaining more information on the composition of cosmic rays at higher energies will provide crucial means to discriminate between the model classes and to understand the origin of the observed flux suppre...