Emphasizing the increasing significance of immigration as a determinant of U.S. population growth, and stressing the need for developing population policy upon which future immigration policy might be based, this report presents projections of U.S. population to determine the impact of immigration in the next 100 years. The projections, given for the years 2000 to 2080, have been calculated under assumptions of constant, increased, or decreased fertility, and varying assumed levels of annual net immigration. It is suggested that (1) the differences attributed to fertility and immigration become more dramatic the longer the projection period; (2) fertility changes are stronger determinants of future population size than immigration changes; and (3) immigration becomes an increasingly important contributor to population growth over time. Also discussed are the effects of fertility and immigration on the future age composition and resident versus immigrant composition of the population, and the implications of the population size alternatives for population and immigration policy. The appendix describes the projection methodology and assumptions. (MJL)