At first glance the destruction of a target cell by a killer cell seems to be a simple endeavor. A closer look, however, reveals the complex mechanisms underlying this task. Killer cells are able to specifically recognize altered or infected cells. A transient contract with target cells has to be established to allow the delivery of lethal molecules or signals. The killer cell then disengages from the damaged cell and moves away to kill other target cells. After the eradication of the target cells, the number or activity of activated killer cells has to be reduced to avoid nonspecific killing...