The Fermi Gamma-ray Space Telescope (formerly called GLAST) is an international and multi-agency space observatory that studies the cosmos in the photon energy range of 8,000 electronvolts (8 keV) to greater than 300 billion electronvolts (300 GeV). An electronvolt is a unit of energy close to that of visible light, so Fermi will catch photons with energies thousands to hundreds of billions of times greater than those we see with our eyes (1 keV = 1,000 eV, 1 MeV = 1,000,000 eV, 1 GeV = 1,000,000,000 eV).

The Fermi mission started in 2008 and carries two instruments: the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM)

The LAT is considered Fermi's primary instrument which detects individual gamma rays. Thereby incoming gamma rays are passing freely through an anticoincidence detector, in which charged cosmic rays are causing a flash of light. Therefore the detector can identify the gamma rays, which then continues through many thin metal sheets until it interacts with an atom contained in one of those foils. In this reaction, called pair production, two charged particles are produced, namely an electron and a positron. Furthermore the LAT contains interleaved silicon strip detectors in which the electrons and positrons create ions and it is therefore possible to recreate their path. In the end, after passing the tracker, the particles are stopped by a cesium iodide calorimeter which measures the total energy of the particles. In combination with the information from the previous detectors the energy and the direction of the gamma ray can be computed. The field of view of the LAT covers about 20% of the entire sky with a resolution of a few arcminutes for the highest energy photons and about 1 degree at 100 MeV.

The GMB complements the LAT in its observations of transient sources and is sensitive to gamma rays as well as x-rays in the energy range of 8 keV to 40 MeV. It detects sudden flares of gamma rays which are produced for example by gamma ray bursts or solar flares. In order to offer a complete view of the sky he scintillators are situated on the sides of the satellite. The design of the detector is optimized for good resolution in time and photon energy.