Provided herein are methods of producing natural killer (NK) cells and NK progenitor cell populations using a two-step expansion and differentiation method. Also provided herein are methods of producing populations of NK cells and NK progenitor cell populations using a three-step expansion and differentiation method. Also provided herein are methods of suppressing tumor cell proliferation using the NK cells, the NK cell populations, and the NK progenitor call populations produced by the methods described herein, as well as methods of treating individuals having cancer or a viral infection, comprising administering the NK cells, the NK cell populations, and the NK progenitor cell populations produced by the methods described herein to an individual having the cancer or viral infection.