The Burst Alert telescope (BAT) is one of 3 instruments on the Swift MIDEX spacecraft to study gamma-ray bursts (GRBs). The BAT instrument is the instrument that first detects the GRB and localizes the burst direction to an accuracy of 1 - 4 arcmin within 10 sec after the start of the event. These locations cause the spacecraft to autonomously slew to point the two narrow-FOV instruments at the burst location within 20 - 70 sec to make follow-up x-ray and optical observations. BAT is a wide-FOV coded-aperture instrument with a CdZnTe detector plane. The detector plane is composed of 32,768 pieces of CdZnTe (4 X 4 X 2 mm), and the coded-aperture mask is composed of approximately 50,000 pieces of lead (5 X 5 X 1 mm) with a 1-m separation between mask and detector plane. The BAT operates over the 10 - 150 keV energy range with approximately 7 keV resolution, a sensitivity of 0.2 ph/cm2-sec, and a 1.4 sr (half-coded) FOV. We expect to detect approximately 300 GRBs/yr for a 3-year mission. The BAT also performs an all-sky hard x-ray survey with a sensitivity of approximately 1 mCrab (systematic limit) and as a hard x- ray transient monitor.