PIP: The value and importance of the concept of natural fertility have been widely debated since Henry's work in the late 1950s. Whereas Henry regarded natural fertility largely as a useful concept for model building, Coale-Trussell demonstrated that this concept could also be used to analyze patterns of age-specific fertility. The set of age-specific marital fertility schedules assembled by Henry and then used by Coale and Trussell involved largely small populations drawn from disparate sources. Nonetheless, from this diverse and fragmentary data, Henry was able to construct a standard of natural fertility which, in the operational form developed by Coale-Trussell, has proved remarkably general in its application and utility. This in turn suggests that the physiological forces shaping fertility are exceedingly strong, especially the incidence of physiological sterility. There appears to be no single universal pattern of natural fertility. Rather, there is a set of closely related age profiles, the exact shape of which is determined by specific behavioral and social factors that influence childbearing. The increase in physiological sterility with age serves to give an upper limit to fertility at each age, while other factors determine how close a given population comes to that maximum. Although no society has ever demonstrated Henry's ideal notion of a purely physiological fertility pattern, the effect of the intervening factors is often weak. In terms of the Coale-Trussell model, maximum likelihood estimation offers the most informative method of implementation and a time series for 1 population is more readily interpretable than a cross-section comparing several populations.