Errors in population forecasts arise from errors in the jump-off population and errors in the predictions of future vital rates. The propagation of these errors through the linear (“Leslie”) growth model is studied, and prediction intervals for future population are developed. For U.S. national forecasts, the prediction intervals are compared with the U.S. Census Bureau's high—low intervals. To assess the accuracy of the predictions of future vital rates, we derive the predictions from a parametric statistical model and estimate the extent of model misspecification and errors in parameter estimates. Subjective, “expert” opinion, so important in real forecasting, is incorporated with the technique of mixed estimation. A robust regression model is used to assess the effects of model misspecification.