Puppis A (G260.4-3.4) is a middle-aged supernova remnant displaying increasing X-ray surface brightness from West to East corresponding to an increasing density of the ambient interstellar medium at the Eastern and Northern shell. The dense IR photon field and the high ambient density around the remnant makes it an ideal case to study in gamma-rays. Three years of Fermi-LAT data have been analyzed showing a spatially extended gamma-ray emission, with a morphology matching that observed in the radio and X-ray domain. The gamma-ray spectrum is well described by a simple power law with an index of 2.1. To constrain the relativistic electron population we have used 7-years of WMAP data to extend the radio spectrum up to 93 GHz. The results obtained in the radio and gamma-ray can be reproduced by both leptonic and hadronic dominated models. However, regardless of the dominant emission mechanism, Puppis A requires a total content of cosmic ray electrons and protons accelerated of at least 1-5% of the initial explosion energy.