Pulsar wind nebulae (PWNe) are the most populous class of sources observed in the TeV energy range, followed by a large sample of unidentified sources (UNID). With its unmatched sensitivity in the GeV energy range, the Fermi Gamma-ray Space Telescope has already detected seven pulsar wind nebulae (PWNe), all of them being linked to a TeV source. In this context, we have analyzed more than three years of Fermi LAT observations to look for GeV counterparts to all 56 potential PWNe (known PWNe + UNID) observed by the TeV experiments. For each of them a gamma-ray flux or an upper limit was derived above 10 GeV. The wealth of multi-wavelength data available and the new results provided by Fermi offer an extraordinary opportunity to constrain the origin of the gamma-ray emission of the large sample of UNID and the radiative processes taking place in known PWNe. We will discuss these new results in detail, focusing on the newly detected sources.