The study of kinship is by now recognized as being one of the key positions in the study of primitive culture. The series of studies to be undertaken by the Institute of the effect upon native culture of modern industrialization will to a great extent centre on the problem of changes in kinship structure, and the changing function of the different types of kinship organization. With this final aim in mind, the following sketch purposely pays attention to the formal aspect only of kinship organization. It goes without saying that such merely formal studies can never be an end in themselves. But there is, on the other hand, always the danger that in historical or functional studies of kinship problems this formal aspect may be unduly neglected. This consideration—that a clear understanding of these formal problems has to precede any attempt at a functional or historical interpretation—is the only reason why these forms of kinship organization as such have been singled out here for theoretical discussion. This discussion does not aim at presenting anything new to the student of kinship organization. It simply attempts to sum up in a form as concise as possible the main results—and the main problems—of modern anthropology in regard to the forms of kinship organization. This can probably best be done by giving definitions of the most important of these forms and discussing their relation to each other.