HASH is a Fortran 77 code that computes double-couple earthquake focal mechanisms from P-wave first motion polarity observations, and optionally S/P amplitude ratios. HASH is designed to produce stable high-quality focal mechanisms, and tests the solution sensitivity to possible errors in the first-motion input and the computed take-off angles. The technique is described by Hardebeck and Shearer (BSSA 92, pp. 2264-2276, 2002.) Examples are provided for data in FPFIT input format. The code is designed to be as input-format independent as possible, so only minor editing is needed to use data in other formats.