Different cellular immune responses are modulated by the cross talk between activating and inhibitory signaling pathways initiated via different cell surface receptors. Similarly, the killing of NK cells is controlled by multiple activating and inhibitory surface receptors. In humans, the major NK triggering receptors, identified so far, include NKp80, 2B4 NKG2D, and CD16 and the natural cytotoxic receptors (collectively named NCRs) include NKp46, NKp44, and NKp30. The two major families of MHC-specific inhibitory receptors identified in humans are the Ig superfamily (KIR and LIR) and the C-type lectin (CD94/NKG2A) receptor superfamily. The different inhibitory receptors show diverse specificity and discriminate between different class I MHC proteins. Much is known about the function and expression patterns of the different NK cell receptors, but the ligand identity of several of the activating NK cell receptors is yet to be discovered. This chapter introduces several research tools that can be used to uncover the identities of different ligands for NK cell receptors.