Microdosimetry is a branch of radiological physics that provides quantitative characterization of the non-uniformity of energy deposition in uniformly irradiated matter. Consideration based on microdosimetry indicate that the action of ionizing radiation on the cells of higher organisms depends on the square of the specific energy absorbed in subcellular volumes. This is the basis of the Theory of Dual Radiation Action. The basic postulate of this theory is reviewed and four factors are discussed that modify its elementary formulation.