This paper reviews literature on yield response of crops to soil compaction. Because soil compaction interacts with several other factors it is difficult to clearly define its effect on yield. In general, increasing soil bulk density is detrimental to crop growth. However, some compaction frequently improves crop production. Usually this occurs in dry conditions and often results from better stand establishment or nutrient uptake. In many situations compaction has little effect on crop yield. This is likely when factors other than those affected by soil density are limiting crop growth.