(or target principle), in radiobiology, one of the first theories of the biological effect of ionizing radiation; it was formulated in the 1920’s and 1930’s.

According to the target theory, living beings have especially sensitive areas, or “targets,” injury of which leads to injury of the entire organism. The discrete nature of radiation and its interaction with matter makes it possible, especially in the case of ionizing radiation, to proceed from the concept of “bombardment” of matter by particles of various energies (photons, fast electrons, and so on) and, in connection with this, from the principle of striking a “target.”

Cells and tissues consist of an enormous number of macromolecules, micelles, fibrils, membranes, and other elements of various structures and sizes. With the doses of radiation used in radiobiology, the probability of a particle or photon striking the rare but vitally important intracellular target (macromolecular and biologically active structure) is small. However, as a result of rare strikes on such a target, even small doses of ionizing rays may cause destruction of the cell or some rare specific reactions in it (for example, mutations of certain genes), whose frequency will increase with the radiation dose.

The target theory is not universal and does not explain all the biological effects that arise under the action of ionizing radiation.