Air reconnaissance over the United Kingdom has been continued during the last four years, a period which has seen a considerable extension of such work, and one notable for exceptionally dry summers. Thus, both the Royal Commission on Historical Monuments for England, through the Air Photographs Unit of the National Monuments Record, and the Royal Commission for Scotland have undertaken extensive surveys, while the growth of local flying has meant that many areas of the country have been reconnoitred more intensively than has hitherto been possible. The account that follows relates almost entirely to the work sponsored by the Committee for Aerial Photography of the University of Cambridge, though information is, indeed, often interchanged with others making similar surveys. The fullest knowledge of any given site comes from study of all available records. The first clue may be obtained by one observer, later reconnaissance by others may amplify the record, perhaps making plain what was previously only suspected. Nevertheless, had it not been for knowledge of the first, perhaps incomplete, observation, subsequent reconnaissance might never have been undertaken. This is a pursuit in which each participant may owe much to others: the cumulative results reflect the activities of many.