This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book provides a collection of methods which can be used to analyze data in most epidemiologic studies. It contains a number of derivations of formulae or, where this would carry too far, some principles as to how derivations could be carried out. The book discusses some general principles which apply to the analysis of epidemiological data and how the precision of an epidemiologic study can best be described. It describes the most important methods used for analyzing epidemiological data. The book begins with an analysis of descriptive data and also discusses the analysis of effect measures that is the measures used to compare exposed and unexposed. It then discusses the multivariate models, dose-response analysis, analysis of the interaction between causes of disease, meta-analysis and computer programs.