This review concerns ticks and mites of known veterinary or medical importance. Acarina that are normally parasitic upon vertebrates include those of greatest importance to the present subject. Parasitism of vertebrates has arisen among quite unrelated forms of Acarina; hence, with the exception of ticks, the families discussed here form a confusing assemblage rather than a natural group. Furthermore, since the tissues of many parasitic Acarina satisfy basic requirements for a wide variety of pathogenic microorganisms, including certain viruses, rickettsiae, bacteria, spirochetes, protozoa, and helminths, one must deal with a similarly wide variety of problems in microbiology. Finally, various degrees of complexity may characterize the ecologic relationships among an acarine species, its vertebrate host species, and a species of microorganism shared by both. For these reasons the prob lems of veterinary and medical acarology, with their breadth of scope, present a stimulating challenge to workers in many fields of basic and applied biology. As an introduction to the great variety of ecologic problems encountered, certain facts are worthy of mention. During at least one stage in their life cycle, all ticks and certain mites are parasitic on or in terrestrial vertebrates. Since some of these parasitic forms attack human beings or domestic animals, they may act as direct causative agents of disease, or they may transmit pathogenic microorganisms. Certain parasitic species, not known to attack man or domestic animals, may play roles in the biological survival mecha nisms of important pathogens by transmitting them among suitable wild hosts and by acting as reservoirs of infection. Some mites which are nor mally free-living may occasionally cause accidental infestations of man, while other nonparasitic forms may harbor the immature stages of parasitic worms. Examples of these and of more complex kinds of relationship will be con sidered in this review. The review itself is selective rather than comprehensive. It aims neither at complete coverage of the literature, nor at a balanced presentation. It is intended to make the reader aware of the breadth of the subject and to pre sent the reviewer's evaluation of certain aspects. The critical reader, appre ciating the scope and diversity of veterinary and medical acarology, will understand this approach, while the bibliographer will find the review of little use.