Background: The pace of aging is a concept that captures the time-related aspect of aging. It formalizesthe idea of a characteristic life span or intrinsic population time scale. In the rapidly developing field of comparative biodemography, measures that account for inter-speciesdifferences in life span are needed to compare how species age. Objective: We aim to provide a mathematical foundation for the concept of pace. We derive desiredmathematical properties of pace measures and suggest candidates which satisfy these properties. Subsequently, we introduce the concept of pace-standardization, which reveals differences in demographic quantities that are not due to pace. Examples and consequences are discussed. Conclusions: Mean life span (i.e., life expectancy from birth or from maturity) is intuitively appealing,theoretically justified, and the most appropriate measure of pace. Pace-standardizationprovides a serviceable method for comparative aging studies to explore differences indemographic patterns of aging across species, and it may considerably alter conclusionsabout the strength of aging.