The articles presented in this special issue both advance what is currently known about treatment implementation in research and practice and illuminate what remains unknown. This article provides a commentary selectively linking current findings to prior research to make arguments regarding those facts that are well established and the gaps in the current knowledge base. The discussion is organized around the themes of implementation in research versus practice, defining and measuring implementation, supporting implementation, and implementation as a moderator of treatment outcome. The commentary concludes by considering major research needs and treatment implementation as fundamental elements of education, psychology, and behavior analysis as applied sciences.