This study relates to the ruins and inhabited towns found in that immense southwestern region composed of the arid plateaus which is approximately bounded on the east by the Rio Pecos and the west by the Colorado River, on the north by Central Utah, and which extends southward to yet undetermined limits in Mexico. The present paper is more directly confined to the ancient provinces of Tusayan and Cibola which are situated within the drainage of the Little Colorado River, and the intention is to follow and supplement it by studies of other typical groups in the region, but the necessary comparisons and generalizations now presented apply to all the varied features which are observed in the remains of Pueblo architecture now scattered over thousands of square miles. The work of surveying and platting in this vast field, together with the consequent coordination of studies and preparation of illustrations, has occupied the author and Mr. Cosmos Mindeleff a large amount of time since the year 1881, though it did not include all of their duties performed during that period. The title of the paper, which only indicates architecture, fails to do justice to the broad and suggestive treatment of the subject. It would be expected, indeed required, that the surveys should be accurate in details and that the physical features of the region should be exhaustively described, but while all this is well done, much more matter of a different though related class, and of great value to ethnology, is furnished. The history, prehistoric and recent, the religion, the sociology and the arts of the people, with their home life and folklore, are studied and discussed in a manner which would be creditable in essays devoted to those special subjects, but are so employed as to be thoroughly appropriate to the elucidation of the general theme. The chapter on the traditional history of Tusayan, which is the individual compilation of Mr. Cosmos Mindeleff, is an important and interesting contribution relative to the history, migrations, and mythology of the people. The traditions are, however, used with proper caution, the fact being recognized that they seldom contain distinct information, but are often of high value from their incidental allusions and in their preservation of the conditions of the past which influenced the lines and limitations of their growth.