NASA’s Double Asteroid Redirection Test (DART) mission, the first planetary defense test mission, deliberately impacted an asteroid in order to change its orbit. By impacting Dimorphos, the secondary member of the Didymos near-Earth asteroid binary system, on 2022 September 26, DART demonstrated asteroid deflection by kinetic impact as a technique that may someday be needed to protect the Earth from an asteroid impact threat. Months of subsequent Earth-based observations of the Didymos system showed that the DART impact changed the binary orbital period by –33 min. The DART impact activated the Didymos system and generated a dust tail that ultimately extended over 70,000 km and that was observed over 10 months after the impact. Dynamical models determined that substantially more momentum was transferred to Dimorphos by the impact than was incident with the DART spacecraft, indicating that the kinetic impact was highly effective in deflecting Dimorphos.