We provide a general introduction to the theoretical and empirical sources informing the development of the Infancy and Early Childhood Section (IEC) of the second edition of the Psychodynamic Diagnostic Manual (PDM-2). We offer a brief exploration of the evolution of developmental psychoanalysis and its applications to infant mental health, along with an example of applying the IEC framework to clinical and developmental data from a longitudinal study based on developmental and psychodynamic principles. This article illustrates the evolution of theory in the context of interdisciplinary integration and explores its implications for diagnosis and clinical formulation.