The purpose of this paper is to recount briefly the work of those biologists who first studied the effect of roentgen rays on living cells, and to show how the results that they obtained, often under great difficulties, have provided the basis of subsequent investigation. Progress in this field has not been steady. During the first dozen years after Röntgen's discovery, the pioneers carried on exploratory work and saw dearly the fundamental problems which should be solved. But in the next dozen years interest in x-ray research waned, although radiotherapy was advancing rapidly. Biologists turned their attention to radium radiations and with their use made important contributions, especially to experimental embryology. Early in the third decade of the present century x-rays again began to be used in biological research and have been increasingly employed ever since. The literature is now extensive. In this review it is possible to cite only a few examples of research in some of the fields of investigation....