Volume 32 of the series addresses one of the most rapidly developing research fields in physics: microcavities. Microcavities form a base for fabrication of opto-electronic devices of XXI century, in particular polariton lasers based on a new physical principle with respect to conventional lasers proposed by Einstein in 1917. This book overviews a theory of all major phenomena linked microcavities and exciton-polaritons and is oriented to the reader having no background in solid state theory as well as to the advanced readers interested in theory of exciton-polaritons in microcavities. All major experimental discoveries in the field are addressed as well. The book is oriented to a general reader and is easy to read for a non-specialist. Contains an overview of the most essential effects in physics of microcavities experimentally observed and theoretically predicted during the recent decade such as:. Bose-Einstein condensation at room temperature. Lasers without inversion of population. Microcavity boom: optics of the XXI century! Frequently asked questions on microcavities and responses without formulas. Half-light-half-matter quasi-particles: base for the future optoelectronic devices.