[en] Radiation effects begin at the cellular level of biological organization. Radiation dosimetry at the cellular level is particularly important for internally deposited alpha and beta particle emitters. Microdosimetry is a mechanism for studying the dose imparted to microscopic sites, for determining hit probabilities, and for determining the probability that sites are missed. Internal microdosimetry calculations are complex, but can be easily executed using computer programs. The investigator must specify the target and its size, determine the radionuclide activity per unit mass for each region in which targets are located, describe the activity per radioactive particulate, understand the geometrical relationship between the activity and the targets, and account for the biological retention of the activity in the region as a function of time. Internal microdosimetry has many potential applications in radiological protection. Microdosimetry is a special research area designed to provide a better understanding of the importance of microscopic patterns of radiation interaction with cells within the broader framework of biochemistry and radiation biology. Its objective is to provide a methodology that is both consistent and precise for correlating biological response to varying levels and distributions of internal emitters. Microdosimetry may contribute to a more complete understanding of the mechanisms of cancer induction by radiation. The correlation between specific energy density and various biological effects might best be treated statistically, since the effects occur in response of stochastic processes. If applied correctly, these concepts should provide a reliable tool for learning more about the effects of radiation and for setting radiation protection standards