Keyfitz has derived an elegant, impressively robust formula for estimating the ultimate size of an initially stable, growing population that abruptly reduces its fertility to replacement level. Reduction of fertility is by the rather unrealistic device of dividing the original age schedule of fertility rates by the net reproduction rate. Only the inertia of the age distribution is thus allowed for, not also that of the fertility schedule, so that the magnitude of population momentum is being understimated, perhaps grossly. The present project retains the key idea of an abrupt imposition of a fixed regimen that is capable in the long run of generating zero population growth, but would make the regimen more realistic. By elaborating the population setting, such different ZPG regimens as reduction of marital fertility by contraception, delayed marriage, raised mortality risks, or permanent net out-migration, may be distinguished. Convergence of the population to stationarity becomes two-phase: a primary adjustment period of changing fertility (or mortality) rates followed by a period of age adjustment. It is the objective of the project to develop the appropriate mathematics and associated computer programs in order to undertake a thorough comparative analysis of population momentum under the four different types of ZPG regimens.