This section describes basic statistical aspects of univariate survival data, and therefore might be skipped by experienced readers. However, it contains notation and important results that form the basis of specific points considered in later chapters. We will use the traditional approach to survival analysis and refer the reader interested in the approach based on counting processes and martingale theory (Aalen 1978) to the well-known books of Fleming and Harrington (1991), Andersen et al. (1993), Martinussen and Scheike (2006), and Aalen et al. (2008).