The discovery of the neutron in 1932, which indirectly led to the potentially catastrophic development of nuclear weapons on the one hand and to the beneficial development of nuclear-generated electricity on the other, has had another, less well-known, but scientifically important consequence: the provision of a uniquely sensitive and pervasive tool for probing condensed matter. Because they are neutral and because they interact mainly with nuclei, neutrons can penetrate into bulk material and provide information that is difficult to obtain with x rays or charged particles. Until recently, nuclear reactors provided the highest-intensity neutron sources. But lately another kind of source is promising to overcome the limitations of reactors. In these sources high-energy protons are made to collide with heavy nuclei, splitting off a large number of neutrons. The process is rather like making chips fly by hitting a rock with a hammer—what the geologists call “spallation.” These neutron spallation sources together with equipment such as that shown in figure 1 have already begun to provide us with useful information accessible in no other way.