Homing and migration differ with regard to the force initiating the flight but are probably alike with regard to the sensory capacities used in guiding the flight. Many hypotheses have been advanced to account for the fact that birds can and do find their way home from considerable distances. These hypotheses are of 4 kinds, each making one or another of the following assumptions: (1) that stimulation emanating from the cote itself directly affects the bird; (2) that the bird is sensitive to slight magnetic variation; (3) that retention of the effects of stimulation received during the outward journey results either in exact retracing of that journey or in an immediate orientation toward the cote; (4) that, upon release, the bird flies without orientation until it encounters stimuli (usually visual) which it had encountered on previous flights, and that these stimuli release those responses which had formerly been successful. The writer insists that no one of these 4 hypotheses nor any combination of them provides an adequate description of all homing behavior. No. 1 and No. 3 may apply to very short homing flights but can hardly apply to those over long distances. There is as yet far more evidence against than for No. 2. The learning theory, No. 4, appears to be more generally applicable than do any of the others. There is a bibliography of 39 titles.