Summary 1.The terms fecundity and fertility (and similarly fecundation and fertilization) have commonly been used interchangeably. 2.Fecundity in the female may be potential (the sum total of ova capable of being produced by the ovary), actual (the ova actually matured and discharged) and observed (the ova of which there is visible evidence, as by the production of eggs or young). The ability to produce offspring is defined by Pearl and Surface as fertility, and in mammals this is the same as observed fecundity. 3.Embryological evidence from a number of forms indicates that, in most if not in all cases, actual fecundity is somewhat greater than the observed. The discrepancy is probably not so great, however, but what the number of offspring produced make a fair measure of fecundity. 4.Observations on multiparous animals (rats) show that the frequency of production of litters increases with the age of the female, at least within limits. This point is not capable of determination in the uniparous domesticated animals, at least on the data available. 5.The evidence also indicates, that in animals in general, there is an increase of litter size with the increasing age of the male. This usually goes to a maximum beyond which there is a decline. In the uniparous forms litter size is more commonly expressed in terms of the relative production of multiple births. to all births. 6.A review of the literature shows more or less fragmentary records on a considerable variety of animals, which all tend to substantiate the above statements. 7.The data on age of dams at time of giving birth to the 747,100 individuals recorded in the American Hereford and Aberdeen-Angus Herdbooks, volumes 1-42 and 1-26 respectively, show that there is at first a relatively rapid increase in fecundity with the advancing age of the female, followed by a more gradual rise, and without indication of a subsequent decline. 8.From the records of age of dam at birth of 1194 lambs in the flock of the University of Wisconsin, a similar curve was obtained, except that in this case it reached its maximum for dams at five years of age, beyond which there was a decided drop. 9.These findings on cattle and sheep coincide with earlier ones on other animals, since all show a general tendency for older females to produce larger litters than younger ones.