Cattle have expanded the KIR gene repertoire, a polymorphic and polygenic im- munoglobulin family that encode Natural Killer cell receptors specific to MHC class I ligands. In humans, KIR are important mediators of innate immunity to viral pathogens such as HCV and HIV, and there is potential for exploiting cattle KIR diversity as a means of improving animal health. Cattle KIR expansion has occurred independently to humans, the result is a cattle KIR haplotype (CKH) with a completely different gene content. Successful sequencing and assembly of the CKH using whole genome techniques has failed. To interrogate cattle KIR, their function and comparative evolution, the content of a CKH must be estab- lished, then the extent of polymorphism and gene presence/absence variation can be studied. In this project the first CKH has been sequenced and assembled using second generation sequencing of BAC clones. This has provided a reference sequence for whole genome sequence data to be aligned revealing the KIR content of different Bovidae species, including the aurochs, the ancestor to all domesticated cattle. Furthermore genome capture and enrichment was performed to determine polymorphic and polygenic KIR variation within 24 different cattle genomes. The sheep KIR haplotype (SKH) was sequenced using PacBio of BAC clones to enable comparative analysis with cattle. The CKH has expanded through block duplications resulting in 16 discrete KIR loci. The haplotype is dominated by functional inhibitory receptor genes and the attenuated remains of activating KIR. Predicted similarity between au- rochs and modern CKH suggests KIR blocks expanded through natural selection and not artificial selection generated through centuries of domestication. Com- parative analysis of the SKH and CKH reveals that sheep have independently expanded at least five of the shared KIR that cattle have expanded. Cattle KIR are extremely polymorphic, with diversity focused within the Ig domains, regions predicted to interact with ligand.