Bulky adducts are DNA lesions generated in response to environmental agents including benzo[a]pyrene (a combustion product) and solar ultraviolet radiation. Error-prone replication of adducted DNA can cause mutations, which may result in cancer. To minimize the detrimental effects of bulky adducts and other DNA lesions, S-phase checkpoint mechanisms sense DNA damage and integrate DNA repair with ongoing DNA replication. The essential protein kinase Chk1 mediates the S-phase checkpoint, inhibiting initiation of new DNA synthesis and promoting stabilization and recovery of stalled replication forks. Here we review the mechanisms by which Chk1 is activated in response to bulky adducts and potential mechanisms by which Chk1 signaling inhibits the initiation stage of DNA synthesis. Additionally, we discuss mechanisms by which Chk1 signaling facilitates bypass of bulky lesions by specialized Y-family DNA polymerases, thereby attenuating checkpoint signaling and allowing resumption of normal cell cycle progression.