This paper presents a discussion of the problem of line insulator contamination by dust and the subsequent performance of the insulators in fog-laden atmospheres. A pioneer investigation of the subject carried out by the Department of Water and Power at Ryan High-Voltage Laboratory of Stanford University is described. Although the investigation was conducted in 1929, the information gained is still recognized as pertinent to the operation of transmission lines today. The quest for a trouble-free insulator for dust and fog areas still goes on while, in the meantime, periodic high-pressure water-stream washing of insulators, which followed the 1929 study of the problem and now is quite common practice, is an essential maintenance function to present operation.