The Burst Alert Telescope (BAT) is one of the three instruments on the Swift MIDEX spacecraft designed to study gamma-ray bursts (GRBs). BAT detects GRBs within 20 seconds and localizes their direction to within 1-4 arcmin within 20 seconds. It triggers an autonomous spacecraft slew to point the two narrow-field-of-view (FOV) instruments (XRT and UVOT) at the burst location within 20-70 seconds for follow-up observations. BAT is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane and a coded-aperture mask composed of lead tiles. It operates over the 15-150 keV energy range with a resolution of ~7 keV, a sensitivity of ~10^-9 erg s^-1 cm^-2, and a 1.4 sr (half-coded) FOV. BAT is expected to detect over 100 GRBs per year during a 2-year mission. It also performs an all-sky hard X-ray survey with a sensitivity of ~2 mCrab and serves as a hard X-ray transient monitor. The Swift mission aims to address key questions about GRB progenitors, multiple classes of GRBs, local environments, and their implications for the early universe. The spacecraft's rapid slewing capabilities and onboard autonomy enable rapid follow-up observations, enhancing our understanding of GRBs. The BAT instrument's design, performance, and operational modes are detailed, including its burst detection, imaging, and location processes, as well as its hard X-ray survey capabilities.The Burst Alert Telescope (BAT) is one of the three instruments on the Swift MIDEX spacecraft designed to study gamma-ray bursts (GRBs). BAT detects GRBs within 20 seconds and localizes their direction to within 1-4 arcmin within 20 seconds. It triggers an autonomous spacecraft slew to point the two narrow-field-of-view (FOV) instruments (XRT and UVOT) at the burst location within 20-70 seconds for follow-up observations. BAT is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane and a coded-aperture mask composed of lead tiles. It operates over the 15-150 keV energy range with a resolution of ~7 keV, a sensitivity of ~10^-9 erg s^-1 cm^-2, and a 1.4 sr (half-coded) FOV. BAT is expected to detect over 100 GRBs per year during a 2-year mission. It also performs an all-sky hard X-ray survey with a sensitivity of ~2 mCrab and serves as a hard X-ray transient monitor. The Swift mission aims to address key questions about GRB progenitors, multiple classes of GRBs, local environments, and their implications for the early universe. The spacecraft's rapid slewing capabilities and onboard autonomy enable rapid follow-up observations, enhancing our understanding of GRBs. The BAT instrument's design, performance, and operational modes are detailed, including its burst detection, imaging, and location processes, as well as its hard X-ray survey capabilities.