Academic Stronger theory on the nature of human well-being is needed, especially as well-being indicators are increasingly utilized in policy contexts. Building on Erik Allardt, who argued that a theory of well-being is, in essence, a theory of human nature, I propose four modes of existence each capturing one dimension central to human well-being: Having recognizes humans as biological creatures requiring certain material resources for survival. Loving captures human social nature and our dependence on others for well-being. Doing highlights the active and agentic nature of human existence. Being acknowledges humans as experiencing their existence. Each mode of existence gives rise to a few more specific needs, and a full assessment of human well-being requires both subjective and objective indicators tapping into these needs. The proposed theory integrates psychological well-being research with sociological and philosophical traditions and contributes to debates about how the progress of nations and sustainability should be measured. Public Well-being is something we all value individually, and it is also a key political goal. Accordingly, how we define and measure well-being influences what physicians, managers, policy-makers, politicians, and international organizations aim to improve through their work. Better theories of well-being make better measurement of well-being possible, which makes possible more effective and evidence-based advancement of human well-being. In this spirit, the present article argues that there are four fundamental dimensions to human well-being: Having highlights that as biological creatures, we have physical needs, loving highlights human social needs, doing highlights that we are active and agentic beings with goals and strivings, and being highlights that we feel and evaluate our lives. To assess well-being, we need measures tapping into all four of these dimensions. And to assess the sustainability of well-being, we need to examine how to provide well-being for all humanity while remaining within planetary boundaries.