INTRODUCTION In 1961 I published a paper on a species of the genus Holothyrus, in which I paid also some attention to the general classification of the mites. Before that time I had chiefly studied Oribatid mites, and I was surprised by the neglect of important discoveries made in the field of Oribatid morphology by specialists of other groups. I realized that a comparative study of acarid morphology was badly needed, and that this could be the only base for a satisfactory general classification. I continued and developed this idea, and in the course of ten years I investigated, in a more or less detailed way, representatives of all groups of mites. In this way I published papers on Opilioacarida (Van der Hammen, 1966, 1968a, 1968b, 1969b, 1971), Holothyrida (1961, 1965, 1968a), Gamasida (1964a), Ixodida (1964), Oribatida (1968), Actinedida (1968a, 1969), Tarsonemida (1970), and Acaridida (1968a). In the same period I published a series of papers on general problems in the field of comparative morphology, ontogeny, phylogeny, and classification (Van der Hammen, 1962, 1963, 1964b, 1968a, 1968c, 1970a, 1970b, 1970c). In addition a small paper was published dealing with some morphological problems in Palpigradida (Van der Hammen, 1969a). The results of this period of comparative study were summarized in a paper read at the Third International Congress of Acarology (Van der Hammen, 1972). In the present paper the subject is treated at greater length, especially by adding detailed diagnoses; the addition of a key for the identification of superorders and orders (although still in its experimental stage) will contribute to the practical value of the system of classification.