Natural killer (NK) cells are lymphocytes of the innate immune system that participate in the elimination of tumor cells. In humans, the activating natural cytotoxicity receptors (NCRs) NKp30, NKp44, and NKp46 play a major role in NK cell-mediated tumor cell lysis. NKp30 recognizes B7-H6, a member of the B7 family which is expressed on tumor, but not healthy, cells. To understand the basis for tumor surveillance by NCRs, we determined the structure of NKp30, a member of the CD28 family which includes CTLA-4 and PD-1, in complex with B7-H6. The overall organization of the NKp30-B7-H6-activating complex differs considerably from those of the CTLA-4-B7 and PD-1-PD-L T cell inhibitory complexes. Whereas CTLA-4 and PD-1 use only the front β-sheet of their Ig-like domain to bind ligands, NKp30 uses both front and back β-sheets, resulting in engagement of B7-H6 via the side, as well as face, of the β-sandwich. Moreover, B7-H6 contacts NKp30 through the complementarity-determining region (CDR)-like loops of its V-like domain in an antibody-like interaction that is not observed for B7 or PD-L. This first structure of an NCR bound to ligand provides a template for designing molecules to stimulate NKp30-mediated cytolytic activity for tumor immunotherapy.