Cellular cytotoxicity is an important effector function for immune reactions of natural killer (NK) cells. Individual NK cells can not only kill a single target cell, but they can eliminate several cells in a serial fashion. This activity is called serial killing. In this chapter we describe the current knowledge about the biology of serial killing NK cells. We introduce the different steps that are necessary for this important effector function. Manipulation of these events may be novel tools to influence serial killing. To analyze this process, different methods have been developed. We introduce these methods and describe their advantages and disadvantages. As NK cell-based approaches of immunotherapy often rely on their cytotoxic activity, we also evaluate the potential use of serial killing NK cells and methods to enhance their activity to further improve these promising therapies.