Extensive fragmentation of nuclear DNA occurs during apoptosis, and the presence of DNA strand breaks is considered to be a marker of the apoptotic mode of cell death. This chapter describes methods to label in situ DNA strand breaks with fluorochromes for detection by flow or laser scanning cytometry. By staining DNA with a fluorochrome of another color, cellular DNA content is measured concurrently and the bivariate analysis of such a data reveals DNA ploidy and cell-cycle phase position of apoptotic cells. The DNA strand break-labeling methodology is also used for detecting the incorporation of halogenated DNA precursors in studies of the cell cycle, proliferation, and DNA replication. In this application, termed "strand breaks induced by photolysis" (SBIP), the cells are incubated with 5-bromo-2'-deoxyuridine (BrdU) to incorporate it into DNA and sensitize the DNA to ultraviolet (UV) light. DNA strand breaks are then photolytically generated by exposing the cells to UV light. The DNA strand breaks resulting from UV-photolysis are subsequently fluorochrome-labeled as for labeling apoptotic-DNA breaks. Because SBIP, unlike the alternative method of detection of BrdU incorporation, does not require subjecting cells to harsh conditions (strong acid or heat) of DNA denaturation, it is compatible with concurrent detection of intracellular or cell surface antigens by immunocytochemical means.