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. (PsycINFO Database Record (c) 2018 APA, all rights reserved)