Major goals of epidemiology are estimation of disease morbidity and mortality and identification and quantification of the impact of risk factors. Age is a recognized risk factor, contributing to the occurrence of multifactorial diseases. Cohort, case-control and cross-sectional observational studies identify age and other putative risk factors and quantify their impact on disease occurrence by estimating relative risks and odds ratios. Ageing per se is not a key concern of epidemiologists, and detailed biological explanations of causal mechanisms may not be offered by the epidemiologist. Nevertheless, the relationships identified in epidemiological studies can offer practical solutions to disease prevention. Age also can confound relationships between disease and other putative risk factors and must be controlled during epidemiological study design and analysis, in order to avoid spurious causal inferences. Additionally, age can modify the effect of other risk factors, necessitating identification of such interactions and the differentiation of effect modifiers from confounders. Comparative epidemiology frequently compares human and animal populations. Meaningful comparisons can only be made by undertaking life span adjustment and age adjustment on animal study data, to address differences between the two populations stemming from different 'biological ages' and age structures, respectively.