The contribution of a ceramic's extrinsic phenomena to its microwave dielectric properties is reviewed. The extrinsic phenomena which are related to the microstructural characteristics, are grouped into four classes: grain characteristics, pore characteristics, phase composition and crystallite characteristics. The individual classes are discussed with the aim of minimizing their mainly detrimental influence on the dielectric losses. A number of examples are described. (C) 2002 Elsevier Science B.V. All rights reserved.