Summary: The recognition of class I MHC molecules by killer cell immunoglobulin-like receptors (KIR) constitutes an integral part of immune surveillance by the innate immune system. To understand the molecular basis of this recognition, the structures of several members of this superfamily have been determined. Despite their functional diversity, members of this superfamily share many conserved structural features. A central question is how these receptors recognize their ligands. The recent determination of the crystal structure of KIR2DL2 in complex with HLA-Cw3 has revealed the molecular mechanisms underpinning this interaction, which ultimately modulates the cytolytic activity of natural killer cells. While the recognition of MHC molecules by KIR is characterized by a number of unique features, some unexpected similarities with T-cell receptor recognition of MHC molecules are also observed. The detailed interactions between KIR2DL2 and HLA-Cw3 and their functional implications will be reviewed here.