The ALICE instrument is a lightweight (4.4 kg), low-power (4.4 watt) imaging spectrograph aboard the New Horizons mission to the Pluto system and the Kuiper Belt. Its primary job is to determine the relative abundances of various species in Pluto’s atmosphere. ALICE will also be used to search for an atmosphere around Pluto’s moon, Charon, as well as the Kuiper Belt Objects (KBOs) that New Horizons is expected to fly by after Pluto-Charon, and it will make UV surface reflectivity measurements of all of these bodies, as well as of Pluto’s smaller moons Nix and Hydra. The instrument incorporates an off-axis telescope feeding a Rowland-circle spectrograph with a 520-1870 Å spectral passband, a spectral point spread function of 3-6 Å FWHM, and an instantaneous spatial field-of-view that is 6 degrees long. Two different input apertures that feed the telescope allow for both airglow and solar occultation observations during the mission. The focal plane detector is an imaging microchannel plate (MCP) double delay-line detector with dual solar-blind opaque photocathodes (KBr and CsI) and a focal surface that matches the instrument’s 15-cm diameter Rowland-circle. In this paper, we describe the instrument in greater detail, including descriptions of its ground calibration and initial in flight performance. New Horizons launched on 19 January 2006.