After the first demonstration of Bose Einstein condensation in the solid state in 2006 and the establishment of exciton polariton condensates in the wider scientific community, an intense interest has been attracted by this phenomenon at both theoretical and experimental level. This book presents in detail the different aspects of fundamental importance related to the polariton condensation. After an overview of the basic concepts for excitons, polaritons and condensates in and out of equilibrium, the book then considers a variety of experimental methods used in their study. A detailed theoretical analysis that supports observed experimental findings is also provided.