  Scattering relativistic electrons with optical lasers can result in a
significant frequency upshift for the photons, potentially producing
$\gamma$-rays. This is what linear Compton scattering taught us. Ultra-intense
lasers offer nowadays a new paradigm where multi-photon absorption effects come
into play. These effects can result in higher harmonics, higher yields and also
electron-positron pairs. This article intends to discriminate the different
laser scenarios that have been proposed over the past years as well as to give
scaling laws for future experiments. The energy conversion from laser or
particles to high-frequency photons is addressed for both the well-known
counter propagating electron beam-laser interaction and for
Quantum-electrodynamics cascades triggered by various lasers. Constructing
bright and energetic gamma-ray sources in controlled conditions is within an
ace of seeing the light of day.
