Mortality refers to death as a study endpoint or outcome. Broader aspects of the study of death and dying are embraced in the term thanatology. Survival is an antonym for mortality. Mortality may be an outcome variable in populations or samples, associated with treatments or risk factors. It may be a confounder of other outcomes due to resultant missing data or to biases induced when attrition due to death results in structural changes in a sample. Mortality is an event that establishes a metric for the end of the life span. Time to death is frequently used as an outcome and, less frequently, as a predictor variable. This entry discusses the use and analysis of mortality data in research studies.