The Galactic Australian SKA Pathfinder (GASKAP) survey is one of several key science projects with ASKAP, a new radio telescope being built in Australia as a technology demonstrator for the Square Kilometer Array (SKA). GASKAP aims to survey about 12,779 square degrees of the Galaxy and the Magellanic System, at high spectral resolution (0.2 km s−1) and using several wavelengths: the λ21-cm HI line, the λ18-cm OH lines, and the comb of recombination lines around λ18-cm. The area covered by GASKAP includes all of the Galactic plane south of declination +40° with |b| < 10°, selected areas at higher latitudes covering important interstellar clouds in the disk and halo, the Large and Small Magellanic Clouds, and the Magellanic Bridge and Stream. Compared with previous surveys, GASKAP will achieve an order of magnitude or greater improvement in brightness sensitivity and resolution in various combinations of beam size and mapping speed matched to the astrophysical objectives.