ABCRATE is a computer program that calculates rate constants by generalized transition state theory (GTST) for atom-diatom reactions with collinear reaction paths. This program is written using the FORTRAN 77 programming language. The program requires a user-supplied subprogram that, given the internal coordinates, can return the potential energy and its gradient at that geometry (sample subprograms for several systems are provided with ABCRATE). The reaction path for dynamics calculations is calculated as the path of steepest descent from the collinear saddle point, which is called the minimum-energy path (MEP). Vibrational modes are treated in curvilinear internal coordinates. The GTST methods include two versions of variational transition state theory (VTST), in particular canonical variational theory (CVT) and improved canonical variational theory (ICVT), and ABCRATE also includes the canonical unified! statistical (CUS) model. All vibrational energy levels are quantized, and anharmonicity may be included by several options, including Morse and Wentzel-Kramers-Brillouin (WKB) anharmonicity for stretches and quartic and centrifugal oscillator anharmonicity for bends. Transmission coefficients to account for tunneling and nonclassical reflection may be included and the methods available for transmission coefficients include the minimum energy path semiclassical adiabatic ground-state method (MEPSAG, also called ZCT), the centrifugal-dominant small-curvature semiclassical adiabatic ground-state method (CD-SCSAG, also called SCT), the large-curvature ground- state method version 3 (LCG3, also called LCT), the least-action ground-state method (LAG, also called LAT), and the microcanonical optimized multidimensional tunneling method (muOMT). Reverse reaction rates are always determined by detailed balance.