GROND -: a 7-channel imager

We describe the construction of GROND, a 7-channel imager, primarily designed for rapid observations of gamma-ray burst afterglows. It allows simultaneous imaging in the Sloan g ' r ' i ' z ' and near-infrared JHK bands. GROND was commissioned at the MPI/ESO 2.2 m telescope at La Silla (Chile) in April 2007, and first results of its performance and calibration are presented.