This paper proposes a conceptualization of the life course as a set of behavioral processes characterized by interdependencies that cross time, life domains, and levels of analysis. We first discuss the need for a systematized approach to life course theory that integrates parallel and partially redundant concepts developed in a variety of disciplines. We then introduce the ‘life course cube,’ which graphically defines and illustrates time-domain-level interdependencies and their multiple interactions that are central to understanding life courses. Finally, in an appendix, we offer a formal account of these interactions in a language that can be readily adopted across disciplines. Our aim is to provide a consistent and parsimonious foundation to further develop life course theories and methods and integrate life course scholarship across disciplines.