High energy gamma-ray astronomy probes the most extreme and violent environments in the Universe. This allows to study, among others, the role of relativistic cosmic particles and to search for dark matter. A review of physics highlights of the current gamma-ray ground-based and space-born detectors will be presented. Prospective studies for the future Cherenkov Telescope Array (CTA), the first gamma-ray observatory in both hemispheres, will also be discussed.