In this book, the authors discuss the occurrences, physical properties and applications of spinels. Topics include optical spectroscopy of transition metals in a-ZnAl2S4 wide band gap semiconductors; development of spinel NiZn ferrites for power field applications; spinel ferrites as catalysts for light alkanes combustion; application of magnesium spinel nanometric coating on aluminum oxide as an ionizing radiation sensor for personal and environmental dosimetry; chromium spinel oxides in high magnetic fields; the structural behavior of natural Cr-bearing spinels compared with synthetic ones; negative temperature coefficient thermistors based on spinel-type ceramics; and synthesis of high surface area MgAl2O4 spinel particles by a new method.