Well-being is a core concept for both individuals, groups and societies. Greater understanding of trajectories of well-being in later life may contribute to the achievement and maintenance of well-being for as many as possible. This article reviews two main approaches to well-being: hedonic and eudaimonic well-being, and shows that it is not chronological age per se, but various factors related to age that underlie trajectories of well-being at older ages. Next to the role of genes, heritability and personality traits, well-being is determined to a substantial extent by external circumstances and resources (e.g., health and social relationships), and to malleable individual behaviors and beliefs (e.g., self-regulatory ability and control beliefs). Although many determinants have been identified, it remains difficult to decide which of them are most important. Moreover, the role of some determinants varies for different indicators of well-being, such as positive affect and life satisfaction. Several prominent goal- and need-based models of well-being in later life are discussed, which explicate mechanisms underlying trajectories of well-being at older ages. These are the model of Selection, Optimization, and Compensation, the Motivational Theory of Lifespan Development, Socio-emotional Selectivity Theory, Ryff’s model of Psychological Well-Being, Self-Determination Theory, and Self-Management of Well-being theory. Also, interventions based on these models are reviewed, although not all of them address older adults. It is concluded that the literature on well-being in later life is enormous, and, together with various conceptual models, offers many important insights. Still, the field would benefit from more theoretical integration, and from more attention to the development and testing of theory-based interventions. This remains a challenge for the science of well-being in later life, and could be an important contribution to the well-being of a still growing proportion of the population.