The spontaneous disintegration of long-lived, naturally occurring isotopes provides one source of information on nuclei. However, only a limited number of nuclei are accessible for study by this natural process and then only under a narrow range of circumstances. On the other hand, nuclear reactigns can be induced in the myriad of pairwise combinations provided by stable or long-lived nuclei and over the wide range of energies provided by the accelerators in the physics laboratories of the world. Reactions therefore provide the greatest volume and widest range of nuclear data. The energy loss of a beam particle can be directly interpreted as an excitation energy in the target nucleus. Usually, however, the data acquire meaning for the structure of nuclei only after they have been interpreted through a reaction theory. The synthesis of such accumulated information into a coherent theory of the nucleus is the main subject of nuclear physics. There are a number of volumes on nuclear structure that review this ngoing process.