A major aim of epidemiologic research is to measure disease occurrence in relation to various characteristics such as exposure to environmental, occupational, or lifestyle risk factors, genetic traits or other features. In this chapter, various measures will be considered that quantify disease occurrence, associations between disease occurrence and these characteristics as well as their consequences in terms both of disease risk and impact at the population level. As is common practice, the generic term exposure will be used throughout the chapter to denote such characteristics. Emphasis will be placed on measures based on occurrence of new disease cases, referred to as disease incidence. Measures based on disease prevalence, i. e., considering newly occurring and previously existing disease cases as a whole will be considered more briefly.