Bicyclic uracils and related compounds are inhibitors of HIV integrase and inhibitors of HIV replication. In one embodiment, the compounds are of Formula I: wherein a, b, Y, R1, R2, R3 and R4 are defined herein. The compounds are useful in the prevention and treatment of infection by HIV and in the prevention, delay in the onset, and treatment of AIDS. The compounds are employed against HIV infection and AIDS as compounds per se or in the form of pharmaceutically acceptable salts. The compounds and their salts can be employed as ingredients in pharmaceutical compositions, optionally in combination with other antivirals, immunomodulators, antibiotics or vaccines.