It is now some twenty years since an audience at St. John's College, Oxford, heard Mr. O. G. S. Crawford describe, with illustrations, some of the pioneer results in the application of air-photography to archaeology. The impression made was that he had not only cleared the air but conquered it; and for a brief hour we looked upon our old world through new spectacles of undreamt power. An entirely new point of view had been gained, and it was evident that we were upon the brink of a revolution in method and attitude in field-research. As time has passed, however, the limitations of the new instrument have become more apparent. Even at first the most striking results were obtained in southern England, and upon prehistoric material. But it is still true that results elsewhere, while often striking, are comparatively meagre. There are, in fact, specific conditions attached to successful archaeological photography from the air; and for everyone, airmen included, the question is first best studied from the ground, since the strongest limitations are imposed by factors on the ground itself.