This sourcebook is an unparalleled resource in the field of family science. It provides a comprehensive overview of both traditional and contemporary theories and methodologies to promote a greater understanding of increasingly complex family realities. It focuses on broad developments in research design and conceptualization, while also offering a historical perspective on developments in family science over time, particularly emerging theories from the past several decades. Each chapter summarizes and evaluates a major theory or methodological approach in the field, delving into its main principles; its debates and challenges; how it has evolved over time; its practical uses in policy, education, or further research; and links to other theories and methodologies. In highlighting recent research of note, chapters emphasize the potential for innovative future applications. Key areas of coverage include: * Risk and resilience, family stress, feminist, critical race, and social exchange theories. * Ambiguous loss, intersectionality, Queer, and family development theory. * Life course framework. * Biosocial theory and biomarker methods. * Symbolic interactionism. * Ethnography. * Mixed methods, participatory action research, and evaluation.