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 interferometer, a review of a single-channel interferometer for generating digital Fresnel holograms of 3-D real-existing objects illuminated by incoherent light, a study on the volumetric changes which occur during photopolymerization under nonisothermal conditions and a discussion of the electrochemical Koesters interferometer technique used for the determination of changes of surface stress of solid electrodes. In order to cope with the unceasing bandwidth demand fueled by the massive use of the Internet, this book includes a review of the two main types of switching devices, namely the SOA-assisted Sagnac switch(TOAD) and the Ultrafast Nonlinear Interferometer. Application of Laser interferometry technique for investigation of acoustic wave fields is considered as well.