[en] The protection of people against the real or possible harmful effects of ionizing radiation reflects a unique combination of efforts involving art, science, engineering and philosophy. The existence of the potential hazard was recognized within a few weeks after the discovery in November 1895, of x rays, the first form of ionizing radiation known to man. Since that early time, almost continuous attention has been directed, in varying degree, to the problem of protecting man against any harmful effects of radiation, with the result that the nature of the problem and its amelioration is probably better understood than for any other toxic agent of such great value to mankind and yet of concomitant possible great harm. Hence is is felt that many valuable lessons may be learned from a detailed understanding of the methods, strategies, mistakes, and successes involved in the development of radiation protection practices that may be usefully applied in cases of other potentially harmful agents. Throughout the development of radiation protection standards, there has been steady growth in the understanding of radiation effects but many uncertainties and important differences of interpretation yet remain. Some of these points of issue will be touched upon, but their evaluation and reconciliation will not be attempted; that is the role of a treatise on radiobiology, since the ultimate solutions bear on many areas other than radiation protection. As events have shown, certain biomedical conclusions have become accepted, within the overall knowledge of the time, only to be radically altered as new knowledge has been developed. Changes in accepted conclusions have importantly influenced the philosophy and practice of radiation protection