A review is presented of mass transfer in the extraction of materials from solid matrices using supercritical fluids (SCFs) as solvents. The purpose is to assemble the body of knowledge relevant to the quantitative aspects of mass transfer leading to mass-transfer fluxes and extraction times. Experimental results are available from studies of extraction of natural substances and fossil fuels and from the regeneration of industrial adsorbents. Factors relevant to extraparticle mass transfer are also discussed and, in addition, an outline of mathematical models of intraparticle mass transfer is presented leading to estimates of single particle extraction times.