Our collective impact on the environment results from a combination of population, affluence and technology. Human population, 2.6bn in 1950, is predicted to reach 10.9bn by 2100. So much we know. But beyond this starting point questions of population are presented in several ways in normative debate, often with problematic underlying moral assumptions. This paper clarifies and critiques four interconnected claims: (1) the Current Carrying Capacity Claim; (2) the Basic Justice Carrying Capacity Claim; (3) the Optimum Population Claim; and (4) the Population Variable Claim. A moral framework for population policy evaluation is then sketched which promises to avoid the most troubling of these moral critiques, whilst maintaining the valuable elements of (2) and (4). This prioritises the ‘morally basic’ at two levels: in terms of specific policy implications and the broader-level need to remain within the circumstances of global and intergenerational justice. However, it faces up to unavoidable trade-offs between other morally significant criteria. Three significant challenges to filling out the model are outlined, and the need acknowledged, however, problematically, for some global-level judgment call.