Demographic projections depict future trends in population size and its distribution by age and sex. Projections are derived using calculations that replicate plausible future changes in a population as a result of fertility, mortality and migration. A central, or middle, scenario depicts a trajectory of change that seems plausible and, overall, more likely than other scenarios. Traditionally, population projections have included two or more alternative scenarios, both to acknowledge the intrinsic uncertainty of the middle scenario and to offer an assessment of the sensitivity of projected trends to changes in underlying assumptions, in particular with regard to fertility. In recent years, statistical models have been used to derive probabilistic assessments of forecast uncertainty. This presentation will examine the most recent set of global population projections by the United Nations (World Population Prospects, 2015 Revision) and will review the state of knowledge about their uncertainty.