Repression of E2F transcription activity by the retinoblastoma (Rb) tumor suppressor through its interaction with the transactivation domain of the E2F transcription factor is one of the central features of G1/S arrest in the mammalian cell cycle. Deregulation of the Rb-E2F interaction results in hyperproliferation, lack of differentiation, and apoptosis, and can lead to cancer. The 2.2-A crystal structure of the Rb pocket complexed with an 18-residue transactivation-domain peptide of E2F-2 reveals that the boomerang-shaped peptide binds to the highly conserved interface between the A-box and the B-box of the Rb pocket in a bipartite manner. The N-terminal segment of the E2F-2 peptide in an extended beta-strand-like structure interacts with helices from the conserved groove at the A-B interface, whereas the C-terminal segment, which contains one 3(10) helix, binds to a groove mainly formed by A-box helices. The flexibility in the middle of the E2F-2 peptide is essential for the tight association of E2F to the Rb pocket. The binding of Rb to the E2F-2 peptide conceals several conserved residues that are crucial for transcription activation of E2F. We provide the structural basis for the Rb-mediated repression of E2F transcription activity without the requirement of histone-modifying enzymes.