A widely accepted assumption in radiobiology is that ionizing radiation kills cells by inducing forms of damage in DNA structures that lead to the formation of lethal chromosome aberrations. One goal of radiation biology research is the identification of these forms of DNA damage, the characterization of the mechanisms involved in their repair and the elucidation of the processes involved in their transformation to chromosome damage. In recent years, evidence has accumulated implicating DNA double stranded breaks as lesions relevant for cell killing. Here, the available information on this topic is reviewed together with the methods most commonly used to quantitate induction and repair of this type of lesion. The presentation concludes with an outline of present research directions and future goals.