Biophysical modeling of radiation-induced DNA damage shows that significant yields of clustered DNA damage are formed after energy deposition by a single radiation track. To date, the majority of studies on radiation-induced DNA damage in cells have concentrated on determination of the yields of single- and double-strand breaks (DSBs), the latter representing one type of clustered DNA damage. It was recognized, however, that clustered DNA damage, which does not contain a DSB, might contain a combination of DNA base lesions and single-strand breaks in proximity. This mini-review discusses some of the recent experimental data confirming the induction of non-DSB, clustered DNA damage by radiation.