The book is devoted to the description of schemes, structures and methods of using multibeam interferometers of the Fabry-Perot type to study processes and phenomena in transparent media located between the mirrors of the interferometer .. Various schemes of multibeam interferometers , are considered Main types of light sources , CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC methods and devices (monochromators) for obtaining narrow spectral lines , Interferometer designs, methods of precise alignment and devices for their implementation , questions of technique of processing of interferograms and ways of their decoding, Methods of recording interference pattern. Methods of using multibeam interferometers for determining voltages in transparent models, quality control of optical parts, plasma diagnostics, studying the parameters of gaseous substances, gas-dynamic processes of rarefied gases, studying thin films, vibrations, etc.. Along with the well-known use of multibeam interference technique for high-resolution spectroscopy and metrology, this book discusses the specifics of using a multibeam interferometer as a device for quantitative study of transparent media. placed between its mirrors. The book is intended for researchers and graduate students engaged in optical interferometry and the study of phenomena and processes in transparent environments and can be useful for senior students of higher educational institutions .