High energy gamma-rays probe the most extreme environments in the Universe, from highly magnetized Galactic neutron stars to distant blazars and high redshift gamma-ray burst emitting GeV photon. The Fermi gamma-ray Space Telescope is rapidly expanding our view of the GeV universe, with a fresh insight into the connection between the GeV and TeV skies. In particular, Fermi observations are confirming findings of the Milagro observatory, the first gamma-ray water Cherenkov detector ever operated. These studies come around during the development of the High Altitude Water Cherenkov (HAWC) observatory, an instrument 15 times more sensitive than Milagro. HAWC, due to become fully operational by 2012, will survey 2/3 of the sky in E>100 GeV photons from its high altitude and low latitude site at Sierra Negra, Mexico, expanding even further our current vision of the high energy sky.