The concept of bidirectionality represents a process of mutual influence between parent and child, whereby each influences the other as well as the dyadic relationship. Despite the widespread acceptance of bidirectional models of influence, there is still a lack of integration of such models in current research designs. Research on bidirectionality could be strengthened through the adoption of advanced methodologies, including behavioral-genetic research designs and advanced structural equation modeling. The aim of this empirical review is to further advance the study of bidirectionality by evaluating the evidence from 25 years of bidirectionality research in infancy and early childhood. The review indicates significant advancements in the use of methods that address the ecological validity of bidirectional effects, as well as areas that continue to rely on correlational designs to detect bidirectional effects. We describe analytic approaches that may improve the specificity of bidirectionality evidence and highlight gene–environment interaction designs as a promising area for future bidirectionality research.