 When using synchrotron x-rays as a source, x-ray microbeam can be obtained by cutting the beam with a precise slit system due to high directionality of synchrotron radiation.Many biological endpoints have been studied including oncogenic transformation, apoptosis, mutations, and chromosomal aberrations.There have been nine international workshops, held approximately once every two years, on Microbeam Probes of Cellular Radiation Response.