DNA double strand breaks (DSBs) are a serious threat to genome stability and cell viability. Accurate detection of DSBs is critical for the basic understanding of cellular response to ionizing radiation. Recruitment and retention of DNA repair and response proteins at DSBs can be conveniently visualized by fluorescence imaging (often called ionizing radiation-induced foci) both in live and fixed cells. In this chapter, we describe a live cell imaging methodology that directly monitors induction and repair of single DSB, recruitment kinetics of DSB repair/sensor factors to DSB sites, and dynamic interaction of DSB repair/sensor proteins with DSBs at single-cell level. Additionally, the methodology described in this chapter can be readily adapted to other DSBs repair/sensor factors and cell types.