Natural killer (NK) cells play an important role in the innate immune response against cancer, in particular in the elimination of tumor metastases and small tumors. NK cell-mediated control of large solid tumors is usually not efficient, although tumors often express high amounts of activating ligands and low levels of inhibitory ligands, such as MHC class I. Thus, we assume that these tumors might be good targets for NK cell-mediated attack. In vitro, NK cells directly kill tumor cells and release soluble factors that affect both innate and adaptive immune responses. To date, in vivo NK cell activation during tumor progression, the influence of the tumor microenvironment on NK cells, and the mechanisms that interfere with their effector function in cancer patients are not completely understood. This review summarizes our current knowledge of NK cells in solid tumors. We will discuss the impact of novel insights into NK cell responses against tumors on the design of NK cell-based therapies.