Professor Alexander Thom, who died in 1985, was a distinguished engineer. Independently of his 'mainstream' academic career, he developed a deep and active interest in the prehistoric megalithic sites of Britain and Brittany, visiting and surveying many hundreds of them over a period of forty years. Thom's interpretations of the field data have aroused strong interest and some intense controversy. The main areas of debate are: geometry (the methods used to set out the megalithic rings, many of which appear to be non-circular); mensuration (the possible use of 'standard' units of measurement in setting out rings and rows); and astronomy (the connection between structures aligned upon the horizon and the rising and setting positions of the sun, moon or stars).