• children's mental health;
  • dissemination and implementation research;
  • evidence-based treatments;
  • treatment integrity

This commentary reflects on issues related to linking the conceptual frame for treatment fidelity presented in the special issue articles to the most current thinking on treatment fidelity within the emerging field of implementation science. The commentary presents several aspects of treatment fidelity in implementation science that generally were not discussed or addressed in the special issue articles, such as the placement of treatment fidelity as only one among several critical implementation outcomes, the relationship of treatment fidelity to design elements such as external validity as well as internal validity, and the importance of considering implementation issues in the measurement of treatment fidelity even in the earlier stages of efficacy and effectiveness research. The excellent in-depth discussion of treatment fidelity in the special series is a promising start to linking this critical concept to the emerging field of implementation science.