This chapter focuses on how we can think about population sustainability and suggests that consideration of population systems should be integral to critical sustainabilities scholarship. Taking a population lens emphasises the relations between population (as collections of people) and planet; how these relations are uneven, socially and spatially; and how these relations may be considered unequal and unjust. At the end of this chapter the reader will know about fundamental ideas about population systems and population change from population studies; key historical and contemporary ways of thinking about population sustainability; trends in population change in different parts of the globe; and challenges of policy responses to population sustainability. This chapter aims to give you an appreciation of how population systems are intertwined with—and, arguably, fundamental to—other meso-level systems that matter for sustainability, including social, economic, and planetary.