This lecture covers the regulation of the ATR-ATRIP complex by RPA in response to DNA damage. It discusses the phosphorylation of Chk1 by RPA, the stimulation of ATRIP binding to ssDNA by RPA, and the recruitment of the ATR-ATRIP complex to ssDNA by RPA. The lecture also explores the in vivo and in vitro defects of yeast mutants in recruiting Ddc2, as well as a model for RPA regulation of ATR-ATRIP in response to DSBs and DNA replication interference.