In this book we describe the collection and analysis of data that speak to relationships between the occurrence of diseases and various descriptive characteristics of individuals in a population. Specifically, we want to understand whether and how differences in individuals might explain patterns of disease distribution across a population. For most of the material, I focus on chronic diseases, the etiologic processes of which are only partially understood compared with those of many infectious diseases. Characteristics related to an individual’s risk of disease will include (1) basic measures (such as age and sex), (2) specific risk exposures (such as smoking and alcohol consumption), and (3) behavioral descriptors (including educational or socioeconomic status, behavior indicators, and the like). Superficially, we want to shed light on the “black box” that takes “inputs”—risk factors such as exposures, behaviors, genetic descriptors—and turns them into the “output,” some aspect of disease occurrence