[en] Recent developments toward the understanding of the radiobiological oxygen effect are discussed. At first the problem of initial lesions is dealt with, various types are compared and their relevance is assessed. It is concluded that at present it is not possible to decide conclusively which of the known radiation induced DNA alterations is responsible for the oxygen effect. Single strand breaks, however, appear to be the least probable. Contributions from cellular sites other than DNA cannot be excluded. In reviewing experiments with various exposure time schedules in different cellular systems it is concluded that the time sequence of the oxygen effect is more complex than usually assumed. There must be at least two radiation chemical reactions, the involvement of secondary radicals is strongly indicated. Their influence depends, however, on the physiology and the structure of the system under investigation. Different biological endpoints are compared with regard to the oxygen effect. It appears that it is linked to DNA function but may be modified by repair processes. A tentative explanatory reaction scheme id psoposed