In the past it has been assumed that if the weekly radiation dose were low enough, recovery and repair would essentially counteract the tissue damage, and the dose accumulated over a long period of time would become less important, in so far as possible injury manifestable in the lifetime of the exposed individual is concerned (i.e., non-genetic effects). It is becoming increasingly evident, however, that in the case of low-level chronic exposure, there is a not negligible accumulation of effect, related to the total dose over a span of many years. On this account, the concept of limiting the accumulated dose (in addition to limiting the weekly dose) has been introduced into the field of radiation protection. Two of the long-term effects of chief concern are shortening of the life span and cancer production. Since it would be of considerable help in setting up permissible limits of exposure to have some idea of the processes involved, the writer has given considerable thought to these problems. In the present paper, mechanisms are described, in general outline, to account for the long-term effects mentioned above. These mechanisms involve many assumptions, which, however, are not unreasonable and may well be used as working hypotheses to be substantiated or discarded as more information accumulates. It turns out that the same mechanisms may account at least in part for the spontaneous aging process and carcinogenesis in man. This paper, therefore, is of a more general nature than indicated by the title. Gross biological effects of x-rays on cells, produced by moderate doses, are influenced by many factors such as oxygen tension, presence of certain compounds (e.g., cysteine), and physiological status of the cell (e.g., time in the mitotic cycle when the radiation is administered, metabolic activity, etc.). Also, in many cases there is some recovery, in the sense that a dose administered over a long period of time is less effective than the same dose given in a short time. On the other hand, when radiation of high specific ionization (e.g., alpha particles, fast neutrons) is used, these modifying factors have little effect on the degree of gross damage produced. This applies also to whole organisms, including mammals. The normal functions of a cell require synthesis of compounds and removal of waste products, involving many steps. It is clearly demonstrated by the study of biochemical mutants that genes control chemical reactions. Therefore, gene mutations brought about by radiation could interfere with metabolic processes. However, ionization can cause chemical changes more directly. It is reasonable to suppose that with moderate or large doses both modes of action are operative.