Microwave resonators are used extensively in telecommunications equipment, including cellular telephones and satellite links, and are at the heart of a multibillion pound market. Many microwave materials are based on or related to the perovskite crystal structure. One such family of microwave materials is Ba6-3xLn8+2xTi18O54 (Ln = lanthanide cation), which has been produced commercially for over 10 years. The extensive, and often contradictory, literature on Ba6-3xLn8+2xTi18O54 is here critically reviewed, with special attention given to its historical development, crystal structure, and the many attempts to tune its properties via doping and variations in processing parameters. Emphasis is placed on the commercially important Nd analogue, although other systems are also considered; and the technologically significant BaO-TiO2 subsystem, which comprises several other electrical ceramics, is revisited to correct many historical inaccuracies.