Readers are introduced to, or re-familiarised with, theories within psychology and sociology relevant to understanding major life transitions within adulthood and older age impacting on health and wellbeing. Consideration is given about the structure and function of the brain responsible for complex human interactions. How Behaviourist theories help explain aspects of learned behaviour is discussed. Social Cognitive Theory and Social Psychology are considered to explain more complex, innovative human actions and wider social processes. These theories along with Interactionism, Feminism, and Functionalism offer a framework for exploring the impact of social structures on wellbeing and role of human agency. Reference is then made to how lifespan and biopsychosocial explanations help capture the dynamic processes of human socialisation at different periods over the life-course.