Scintillator materials are widely used for the detection of ionizing radiation in a variety of applications including high energy and nuclear physics, astrophysics, geophysical exploration, medical imaging, security inspection, and industry. Research involving the use of photoluminescence, radioluminescence, thermoluminescence, and synchrotron radiation is underway to further our fundamental understanding of the factors affecting scintillator performance and to discover improved materials. The importance of defects and their effects on energy transfer and overall scintillation efficiency are illustrated by considering recent results for cerium-activated crystals and glasses.