The present chapter focuses on establishing Werner’s developmental psychology concepts based on William Stern’s Critical Personalism. This involves not only highlighting the early collaboration between Stern and Werner but also underscoring a deeper intellectual affinity between them. However, this affinity is only implicitly present in Werner’s seminal work, Comparative Psychology of Mental Development. The current chapter makes this connection explicit and demonstrates how Werner’s core concepts of hierarchical integration and differentiation of psychological functions can benefit from a more comprehensive association with Critical Personalism. In doing so, Werner’s ideas are reinterpreted, leading us to the central concept of personalistic conditional genetic analysis, through which we aim to analyze the development and transformation of psychological functions (here, action and personality), provide explanations, and generate knowledge for interventions. This is not only significant for the history of psychology but also serves as a foundation for an updated developmental psychology grounded in personalistic principles.