Recent advance in immune checkpoint therapy targeting T cell inhibitory receptors has opened a new avenue for cancer immunotherapy. However, one major limitation of T-cell killing is the inability to recognize and eliminate HLA-I negative tumor cells. NK cells are important innate immune cells that primarily target HLA-I negative tumor cells and, therefore, might overcome resistance to T-cell-based immunotherapy. While a number of NK-cell-based therapies are under active investigation, the functional interactions between NK cells and tumor cells and between NK cells and anti-cancer treatments are not fully understood. Developing a standardized phenotypic and functional analysis of NK cells in the tumor microenvironment and periphery is a prerequisite for NK cell research. In this collection, we would like to call for outstanding works that illustrate experimental approaches to reveal the phenotypic changes and potential molecular mechanisms of NK cells in the context of tumor development and anti-cancer treatments. These may include but are not limited to the assessments of NK cell development and maturation, NK cell activation and tumoricidal capability, NK cell polarization, NK cell exhaustion, NK cell checkpoint, NK cell functional modulations by treatments such as chemotherapy, radiotherapy, targeted therapy, and immunotherapy, and modification of NK cell activities using immunoadjuvants. All conventional and emerging high-throughput experimental strategies using in vitro, ex vivo, and in vivo models are encouraged. These findings will have important implications for improving NK cell research methodology, gaining insight into NK cell phenotyping and functional depiction, and promoting the application of novel NK-cell-based anti-cancer therapies.