Cells activate complex signaling networks in response to DNA damage. Several proteins and protein complexes are involved in sensing DNA lesions and initiating the DNA damage response networks. The subsequent DNA damage responses, including the initiation of DNA repair pathways, the activation of cell cycle checkpoint controls and the induction of apoptosis, help maintain genomic stability in mammalian systems. Failure to establish the appropriate DNA damage signaling networks results in genomic instability, which is a known causal factor in tumorigenesis. This review will discuss recent progress in the understanding of the mechanisms by which mammalian cells sense DNA lesions and transduce DNA damage signals during early DNA damage responses.