Insulator contamination has been one of the significant subjects in the insulation design of power transmission equipment. The insulator designs and countermeasures against contamination have been studied all over the world, taking both environmental and climatic conditions into consideration. Advanced technology in insulator contamination designs have been developed. A polymer insulator, which is light in weight and has excellent contamination performances, and a semi-conducting glaze insulator, which has high corona suppression performance and high contamination withstand voltage, have been successfully developed.