  The Modular X- and Gamma-ray Sensor (MXGS) is an imaging and spectral X- and
Gamma-ray instrument mounted on the starboard side of the Columbus module on
the International Space Station. Together with the Modular Multi-Spectral
Imaging Assembly (MMIA) (Chanrion et al. this issue) MXGS constitutes the
instruments of the Atmosphere-Space Interactions Monitor (ASIM) (Neubert et al.
this issue). The main objectives of MXGS are to image and measure the spectrum
of X- and $\gamma$-rays from lightning discharges, known as Terrestrial
Gamma-ray Flashes (TGFs), and for MMIA to image and perform high speed
photometry of Transient Luminous Events (TLEs) and lightning discharges. With
these two instruments specifically designed to explore the relation between
electrical discharges, TLEs and TGFs, ASIM is the first mission of its kind.
