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An ion microbeam is a very narrow beam of charged particles, typically protons, alpha particles, or heavier, of micrometer/submicrometer size, corresponding to cellular/subcellular dimensions. Together with integrated techniques for locating live cellular or subcellular targets, they allow rapid sequential irradiation of these targets. This review covers both the technology involved in modern single-cell microbeams, as well as some current applications. The recent explosion of interest in microbeams was initially driven by interest in the domestic radon problem, in which target cells are exposed either to zero or one alpha particle. Microbeams allow cells to be individually irradiated with exact numbers of particles. As microbeams were built, refined, and used, the biological questions that were addressed with them have considerably broadened, to encompass many aspects of damage signal transduction. Two areas in particular have attracted much interest: One is the use of microbeams to address the sensitivity of subcellular targets, such as the cytoplasm or mitochondria. The other reflects the ability of the microbeam to irradiate some cells, but not others, allowing a direct investigation of the so-called bystander effect, where signals from irradiated cells can apparently cause biological responses in neighboring unirradiated cells.