Provided are antibodies that specifically bind to Programmed Death-1 (PD1,Pdcd-1, orCD279) and inhibit PD1-mediated cellular signaling and activities in immunecells, antibodies bindingto a set of amino acid residues required for its ligand binding, and uses ofthese antibodies to treat ordiagnose cancer, infectious diseases or other pathological disorders modulatedby PD1-mediatedfunctions.