In this chapter, we review and evaluate the three primary sources of measures in research on socioemotional development: observational methods; verbal reports, including interviews and questionnaires; and psychobiological approaches, including neurophysiological, psychophysiological, and genetic assessments. Issues of reliability and validity are discussed along with the advantages and disadvantages of each approach to measurement. Illustrations are provided for children of different ages and issues unique to each class of measurement are also considered. The chapter begins with a discussion of common issues that cut across measurement approaches, some of which are unique to developmental science; these include measuring continuity and discontinuity, stability and change, and the roles of participant characteristics and measurement contexts. The chapter closes with a discussion of future opportunities and challenges, including the adoption of measurement approaches that capture complex transactions across levels of analysis from genes to brains to behavior; neuroimaging of social interaction; and the triangulation of methods and measures across observations, verbal reports, and psychobiology to provide a “molecules‐to‐mind” approach to the study of socioemotional development.