The physiology of drought resistance has been reviewed quite extensively. These reviews and additional sources of information will be used to crystallize the essential and relevant information toward plant breeding. In some respects the physiology of drought resistance is open to controversy, while in other respects knowledge is just emerging and is far from being put to use in plant breeding. Genetic variations in yield response to drought stress were found in many cases when measurements were made in terms of yield under stress, the ratio of yield under stress to yield under nonstress conditions, stress/nonstress yield normalized for phenology, a "stress index". Evidence for genetic variation in various components of dehydration avoidance is ample. Selection for drought resistance by any selection index requires a rigorous control over the stress environment. Selection indexes that can be used in any moisture environment would be ideal, but they remain to be perfected and applied.