Purpose of the review: In recent years we have been able to see the different Killer cell immunoglobulin like receptor (KIR) mismatching model with corresponding to their receptors and their ligands. Starting from the ligand-ligand mismatch to the KIR genes tools to know more about how the mismatching works and to discover more about how their mismatching can be benefited in the Hematopoietic Stem Cell Transplant (HSCT) outcome. This review serves the purpose of knowing their limitation and benefit after KIR Mismatching and how every model served their purpose and how they lead to the development and discovery of new model and their role as a better and worse outcome in HSCT, and what the future opportunity holds for these KIR . In this review we will learn about KIR and HSCT, how they help in selection of donor on the basis of KIR mismatching and how they impact on HSCT outcome. Recent findings: Most of the researches and study have shifted themselves towards learning about KIR genes and how it benefits in HSCT. Summary: Current and past researches have shown beneficial role of KIR mismatching and their role in HSCT outcome.