An interferometer is a device that splits a beam of light into two beams. In this book, the most prominent active interferometers are discussed from a research point of view focusing on three variants. Among the many topics of research included in this book are: results of adaptive interferometers operation based on photorefractive crystals and the effect of the non-steady state photo-electromotive force, a reanalysis of the framework of the 'magnetic' components of gravitational waves, the development of a real-time in situ surface height measuring system based on phase shifting...