Recently, scholars’ calls for psychology to move beyond traditional ways of conceptualizing and researching human behaviour have increased. Contextual Action Theory (CAT) is a conceptual, methodological, and practical framework that is consistent with these calls. In CAT action is conceptualized as a complex, dynamic, and multidimensional system that can be comprehended from multiple levels of organization and from multiple perspectives. The Action-Project Method (A-PM) is the research method that emerged from CAT. It is a qualitative, naturalistic, process-oriented, longitudinal methodology that has been used to study joint, goal-directed actions and projects. The focus of data gathering and analysis is the thorough description and understanding of people’s actions as they are constructed in daily living. To illustrate how CAT has been used to conceptualize the design of research and the A-PM to reflect that design, we present an integration of findings from several recent studies on the transition to adulthood that used the theory and the method.