PIP: Natural fertility populations provide a benchmark against which one can gauge controlled fertility. The author bridges the gap between demographers who are interested in fertility and reproductive biologists who are concerned with the determinants of fecundity, the capacity for reproduction. The delineation of fecundity enhances the ability to translate reproductive physicological research into predicted effects on fertility, as well as the emphasis on fetal loss as a major component of previously unspecified onset of permanent sterility. Chapters cover the following topics: introduction, patterns of natural fertility, fecundity definition, fecundability (fecundability and the distribution of fecund waits, the empirical estimates of fecundability, age and fecundability, physiological determinants of fecundability, a model of fecundability), fetal loss )expected number of fetal deaths, length of gestation ending in loss, duration of infecundability following each fetal loss, and the total impact of fetal loss), the male contribution to fecundity, and conclusions. Fecundity reflects a woman's ability to conceive and her ability to carry the pregnancy to term. The components of the ability to conceive are 1) susceptability factors (menarche, menopause, lactational anovultion, and pathological sterility), and 2) fecundability factors (net of coital frequency): length of ovarian cycles, proportion of cycles ovulatory, duration of the fertile period, and probability of conception from a single insemination in the fertile period). The ability to carry a pregnancy to term is dependent on the 1) probability of fetal loss/conception, length of gestation ending in fetal loss, and the duration of residual infecundability following fetal loss. The paper examines how fecundity affects the age pattern of natural marital fertility. A homogeneous fecundity model is developed to show the relationship between fecundity total and the distribution of fecund waits, a period between conceiving and conception with regular unprotected intercourse. However, in a heterogeneous model it may be assumed that apparent fecundability varies according to Henry's beta distribution, with human variation ranging from .14 to .31 and iwth adjustments for fetal loss .17 - .38. Heterogeneity increases expected fecund time to conception by about 50%, or 4-9 months before conceiving. The age pattern of fecundability shows low levels in adolescence with a rapid rise to a peak of 25 years and declines thereafter due to coital frequency. The Wood and Weinstein model of fecundability with links to coitus, physiology and the probability of conception is also discussed.