Randers-Pehrson, G., Geard, C. R., Johnson, G., Elliston, C. D. and Brenner, D. J. The Columbia University Single-Ion Microbeam. Radiat. Res. 156, 210–214 (2001). A single-ion microbeam facility has been constructed at the Columbia University Radiological Research Accelerator Facility. The system was designed to deliver defined numbers of helium or hydrogen ions produced by a van de Graaff accelerator, covering a range of LET from 30 to 220 keV/μm, into an area smaller than the nuclei of human cells growing in culture on thin plastic films. The beam is collimated by a pair of laser-drilled apertures that form the beam-line exit. An integrated computer control program locates the cells and positions them for irradiation. We present details of the microbeam facility including descriptions of the collimators, hardware, control program, and the various protocols available. Various contributions to targeting and positioning precision are discussed along with our plans for future developments. Beam time for outside users is often available (see www.raraf.org).