A new generation of gamma-ray detectors has been developed, primarily to study the structure of atomic nuclei. Much of the knowledge about the structure of nuclei, as well as atoms and molecules, has come from studying their energy levels through the detection of electromagnetic radiation emitted when the system makes a transition from one state to another. The electromagnetic radiation from atoms and molecules has an energy around 1 electronvolt, while for nuclei it is of order 10⁶ eV (1 MeV). Thus, for the study of nuclei it is necessary to design and build detectors aimed at an energy range which is quite different from that of the typical photomultipliers used for detecting electronvolt photons. The recent detectors incorporate new technologies and large arrays to provide high resolution and high efficiency, resulting in an improvement of about a factor of 100 in the ability to resolve rare features of nuclear structure.