The Gamma Ray Large Area Space Telescope (GLAST) mission is a followup to the successful EGRET experiment onboard the Compton Gamma Ray Observatory (CGRO). It will provide a high‐sensitivity survey of the sky in high‐energy &ggr;‐rays, and will perform detailed observations of persistent and transient sources. There are two experiments onboard the GLAST — the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM).The primary mission of the GBM instrument is to support the LAT in observing &ggr;‐ray bursts (GRBs) by providing low‐energy measurements with high time resolution and rapid burst locations over a large field‐of‐view (⩾ 8 sr). The GBM will complement the LAT measurements by observing GRBs in the energy range 10 keV to 30 MeV, the region of the spectral turnover in most GRBs. An important objective of the GBM is to compute the locations of GRB sources on‐board the spacecraft and quickly communicate them to the LAT and to the ground to allow rapid followup observations. This information may be used to re‐point the LAT towards particularly interesting burst sources that occurred outside its field‐of‐view.The GBM consists of 14 uncollimated scintillation detectors coupled to phototubes to measure &ggr;‐ray energies and time profiles. Two types of detectors are used to obtain spectral information over a wide energy range: 12 NaI(Tl) detectors (10 keV to 1 MeV), and 2 BGO detectors (150 keV to 30 MeV). The detectors are distributed around the GLAST spacecraft to provide a large, unobstructed field of view. The 12 NaI(Tl) detectors are mounted with different orientations for use in locating GRB sources. © 2004 American Institute of Physics