I will give a short overview of high energy diffuse gamma-ray emission measured by the Fermi LAT and H.E.S.S. experiments. The talk will consist of two parts. In the first part I will talk about the diffuse emission itself, what it can teach us about Galactic cosmic rays (CRs), the search for sources of PeV CRs, populations of faint Galactic and extragalactic sources. In the second part I will talk about the use of diffuse emission measurements in setting limits on new physics, such as dark matter annihilation and evaporation of primordial black holes.