--- - |2+ Background Scientists have developed evidence‐based interventions that improve the symptoms and functioning of youth with psychiatric disorders; however, these interventions are rarely used in community settings. Eliminating this research‐to‐practice gap is the purview of implementation science, the discipline devoted to the study of methods to promote the use of evidence‐based practices in routine care. Methods We review studies that have tested factors associated with implementation in child psychology and psychiatry, explore applications of social science theories to implementation, and conclude with recommendations to advance implementation science through the development and testing of novel, multilevel, causal theories. Results During its brief history, implementation science in child psychology and psychiatry has documented the implementation gap in routine care, tested training approaches and found them to be insufficient for behavior change, explored the relationships between variables and implementation outcomes, and initiated randomized controlled trials to test implementation strategies. This research has identified targets related to implementation (e.g., clinician motivation, organizational culture) and demonstrated the feasibility of activating these targets through implementation strategies. However, the dominant methodological approach has been atheoretical and predictive, relying heavily on a set of variables from heuristic frameworks. Conclusions Optimizing the implementation of effective treatments in community care for youth with psychiatric disorders is a defining challenge of our time. This review proposes a new direction focused on developing and testing integrated causal theories. We recommend implementation scientists: (a) move from observational studies of implementation barriers and facilitators to trials that include causal theory; (b) identify a core set of implementation determinants; (c) conduct trials of implementation strategies with clear targets, mechanisms, and outcomes; (d) ensure that behaviors that are core to EBPs are clearly defined; and (e) agree upon standard measures. This agenda will help fulfill the promise of evidence‐based practice for improving youth behavioral health. - Journal of Child Psychology and Psychiatry, EarlyView.