The necessity for fruit thinning is generally accepted. While limited hand thinning is still practiced in some areas, Northeast fruit growers rely almost entirely on chemical thinning. In some years, as much as 85 per cent of the apple acreage in New York State is chemically thinned. This practice is inexpensive and effective, but the results are not always totally satisfactory. Complete failures (little or no thinning or drastic overthinning) are rare, but the variability in results may exceed the limits of expediency. The factors that contribute to this variability can be conveniently divided into six categories as follows: initial fruit set, variety, materials and rates, time of application, physical factors that affect the application of the material and its absorption, and physiological factors that affect the response of the trees.