By killing infected host cells, cytotoxic T lymphocytes (CTL) mediate an important defense mechanism against viruses and other intracellular pathogens. Quantitative aspects of this killing process have been studied for several decades in vitro. More recently, methods have been developed to measure the timescales of CTL killing in vivo. Here, we review the estimates of kinetic rates involved in CTL killing which were obtained in these studies, and elaborate on the differences between them.