Natural killer cells were first described in the early 1970s on a functional basis according to their ability to lyse tumor cells in the absence of prior stimulation. Since their discovery, NK cells have been shown to have other important functions that led to the use of NK cells as a form of adoptive immunotherapy. Over the next 5-10 years, with the advances in the field of NK cells we will undoubtedly see the use of allogeneic and autologous NK cells at the forefront of cancer immunotherapy.