Our current knowledge of NK-cell recognition and effector function suggests that it will be possible to design various new NK-cell-based immunotherapies against human cancer. The application of NK cells is already showing promise using HLA-mismatched haematopoietic stem cell transplantation for treatment of haematological malignancies. A better understanding of NK-cell heterogeneity and function will only broaden the applications for human cancer. Here we review the key developments that will propel this field.