The analysis of lifetime data (or more exactly, time-to-event, event-history, or duration data) plays an important role in medicine, epidemiology, biology, demography, economics, engineering, actuarial science, and other fields. It has expanded rapidly in the last three decades, with works having been published in various disciplines in addition to statistics. But what distinguishes survival analysis from other fields of statistics? Why does survival data need a special statistical theory? The main problem is censoring, which means that, for some individuals in the study population, the researcher only has the information that the event of interest did not occur before a particular time point.