Unlike individuals on Earth who are protected against most types of radiation by the shielding characteristics of the atmosphere and the electromagnetic field, astronauts aboard a spaceship or on the surface of the moon or another planet will receive much higher doses of ionizing radiation. Besides this, the radiation quality present in space is very different from the radiation on Earth. Whereas typical terrestrial radiation mainly consists of low-LET radiation such as X and gamma rays, space radiation is comprised of high-energy protons and high-charge (Z) and energy (E) nuclei. Therefore, for manned spaceflight, the biological response after cosmic ray exposure is of critical concern for risk assessment for astronauts (Legner 2004). Ongoing research focuses on the identification of (predictive) biomarkers to determine the radiosensitivity of an individual. This chapter reviews some of the most frequently used cytogenetic assays. In addition, emerging technologies for high-throughput screening of interindividual differences in radiosensitivity are discussed.