Technical Abstract: The major source of soil compaction in soybean production in the Midwest is the wheel traffic of farm machinery, especially when the soil is relatively wet and has a low bearing capacity. Wheel traffic of farm machinery with axle loads less than 5 tons (most tractors) will cause compaction mainly in the surface 30 cm of the profile. This paper reviews and summarizes the findings of 52 location years of field research in Minnesota on clay loam soils, studying the effects of compaction on soybean growth and yield. If the growing season (defined as May-August) precipitation is more than about 380 mm, soybean yields may be decreased by up to 20% as a result of compaction in the surface 30 cm of soil. If the precipitation is less than 380 mm, yields may be increased up to 40% by this surface compaction. These yield responses appear to be due mainly to soil water status rather than mechanical soil strength. If the axle load of the field machinery is more than 10 tons, bulk density, hydraulic conductivity and penetrometer resistance can all be significantly changed to a depth of 60 cm. These changes in deep soil conditions appears to be more detrimental for corn growth than for soybeans, probably because the later has a tap root system. Since corn and soybeans are commonly grown in rotation, subsoil compaction during soybean harvest can be important, even though it may not adversely affect soybean production itself. These data are useful in developing guidelines for farm managers using heavy farm machinery so as to avoid or minimize the detrimental effects of soil compaction while at the same time taking advantage of the time-savings aspects of large equipment.