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Interpretations of radiation effects frequently call upon the word “threshold.” In this letter we wish to explore the meaning of this word, its relationship to the fundamental character of radiation effects, and to the question of perception. We make no distinction between “stochastic effects” and “non-stochastic effects” (ICRU 1971, ICRP 1977). As conceived here, all interactions of radiation with matter are probabilistic. At the cellular or molecular level, where most radiation effects originate, the statistical nature of ionization and excitation events gives rise to considerable fluctuation in the number of these primary events in sensitive sites. Whatever the array of primary events required in a sensitive site to initiate an observed end-point, the statistical character of these events argues against the existence of a threshold. In this sense, there are no non-stochastic processes. The severity of an effect in tissue is a measure of the probability of occurrence of the initiating event in the cells making up that tissue. There may be, however, a quasi-threshold of perception, associated with the emergence of detected events related to the radiation field above a background of events unrelated to that field. In particular, we propose that such questions as the possible existence of a low-dose threshold for cancer induction, or of a low-let threshold for the activation of a sensitive target, must, a priori, be answered in the negative.