This article introduces ‘existence theory’ as a new approach to sociological theory and research. Existence theory starts from the assumption that people organise their lives around a limited set of existential milestones. Cultural expectations are such that without the accomplishment of those milestones, individuals may experience their lives as incomplete. Examples of milestones can include the attainment of formal education, a lasting partnership and the creation of a family, but in general the milestones which are important to individuals and their precise articulation will depend on a variety of cultural and structural factors. The achievement of existential milestones often depends on that of other existential milestones, thereby producing what we call an ‘existential ladder’. The article also elaborates on the significance of ‘existential urgency’ in that, due to a variety of factors (some biological, some cultural and structural), there are time limits on when certain existential milestones ought to be achieved by. In contemporary society, we note that individuals seem to have more choice about which milestones are important to them and when they can be achieved, although we emphasise that this flexibility is unevenly distributed. This then provides a steppingstone towards an elaboration of the power dynamics and inequalities underlying both the experience and the achievement of existential milestones. Finally, this paper shows how existence theory helps to reflect on a variety of social phenomena of contemporary significance: populism in politics, forced migration, and the coronavirus pandemic.