The soybean cyst nematode (SCN) is a serious, widespread pest of soybean in Iowa and most soybean-producing areas of the Midwest. The nematode infests more than 70 percent of the fields in Iowa. However, SCN usually causes no obvious aboveground symptoms for many years after being introduced into a field. Consequently, many SCN-infested fields in Iowa may go undiagnosed. The lack of symptoms and subsequent missed diagnosis are unfortunate because the key to effective management of SCN is early detection, before large nematode population densities develop. It is much easier to keep low population densities low than to drive high population densities down. Large nematode population densities can cause severe damage to soybeans, especially in very dry years.