Microwave dielectrics have been researched up to present, and many compounds with excellent properties are presented, which are listed in a book ”Dielectric Material for Wireless Communication” written by Sebastian (2008). These materials are expected for usage of millimetrewave applications and new applications as paraelectrics, so origins of losses for microwave dielectrics are resumed based on the polars produced by impurities, defects, grain boundary, and stress originated from fluctuation of lattice constants. For new applications, the materials with low losses should be redesigned.