Auger-electron cascades with their ability to deposit energy in extremely small volumes, typically in the range of cubic nanometers, have served as valuable probes of radiobiologic phenomena. Results from their experimental use form part of the evidence that nuclear DNA is the most radiosensitive cell element; that chromosomal aberrations and large scale double-strand breaks are correlated with reproductive survival; that neoplastic transformation and also mutagenesis are greatest at low doses with high specific ionization; and that, like high linear-energy-transfer radiation, Auger-electron cascades can lead to bystander effects. We have also learned that radiobiologic responses to Auger-electron emission are particularly sensitive to the site of decay, not only within the cell but also in the nucleus within the fine structure of chromatin.