Book excerpt: An optical cavity confines light within its structure and constitutes an integral part of a laser device. Unlike traditional gas lasers, semiconductor lasers are invariably much smaller in dimensions, making optical confinement more critical than ever. In this book, modern methods that control and manipulate light at the micrometer and nanometer scales by using a variety of cavity geometries and demonstrate optical resonance from ultra-violet (UV) to infra-red (IR) bands across multiple material platforms are explored. The book has a comprehensive collection of chapters that cover a wide range of topics pertaining to resonance in optical cavities and are contributed by leading researchers in the field. The topics include theory, design, simulation, fabrication, and characterization of micrometer- and nanometer-scale structures and devices that support cavity resonance via various mechanisms such as Fabry–Pérot, whispering gallery, photonic bandgap, and plasmonic modes. The chapters discuss optical cavities that resonate from UV to IR wavelengths and are based on prominent III-V material systems, including Al, In, and Ga nitrides, ZnO, and GaAs.