With pressure and vibration on a soil, air spaces between soil particles can be reduced by displaced soil particles. Activity associated with heavy machine traffic increases the density of the soil and can also increase the resistance of the soil to penetration. This paper reviews research related to disturbance of forest soils with a primary focus on compaction in ash-cap soils. The general process of compaction is described along with physical properties of ash-cap soils that relate to compaction. Ash-cap soils have physical soil properties most closely aligned to silt-loam soils. Undisturbed ash-cap soils often have low bulk densities to variable depths. Under moisture conditions near field capacity, these soils are susceptible to significant disturbance from machine traffic, and when the disturbance causes increases in bulk density, the soils are not likely to recover from this disturbed condition for many years. Machine traffic on forest soils generally occurs as a result of some form of active stand management including precommercial thinning, intermediate and final timber harvest, and site preparation activities involving slash disposal and treatment. The direct contact between equipment and the forest soil will result in some type of soil disturbance. The degree and extent of the soil disturbance is most often controlled through guidelines on the selection and operation of equipment and by restricting the location and operating season for equipment.