Stress is defined and progress in studies of physiological stress in sweet potato is reviewed, with emphasis on breeding. Sweet potatoes were bred in the heavy clay soils of Mayaguez by selecting seedlings that had grown in such soil and by polycrossing. The population so developed increased in tolerance to the soil, and outstanding clones were found. These clones were shown to have considerable yield stability in better soils. Seedlings were also grown in pots in the greenhouse and subject to several stresses: flooding, drought, soil acidity, shade, salinity, poor fertility, and competition with other plants. Selections for stress resistance were made and selections were polycrossed. Stress resistance increased in populations and outstanding clones were produced that outyielded standard clones in field trials under stress. Stresses reduced plant growth and resulted in some cases in characteristic responses to stress. Components of yield were explained. Increased yields occurred chiefly due to increased storage root size. Selection for stress results in simultaneous selection for genes for high yield, and for resistance to the stress in question. Prospects for further progress are excellent.