Improving the quality of teaching and instruction takes time and planning and may be costly and require intensive work. This is particularly true when trying to make improvements among multiple preschool sites, because preschool systems often lack a cohesive infrastructure for supporting high-quality implementation of curricula and teacher practices. This brief provides guidance on implementing supplemental classroom curricula and can help preschool administrators consider how to direct their efforts to support teaching and learning. Evidence built from 15 years of MDRC research on implementing supplemental curricula in preschools highlights important lessons for promoting classroom quality, instructional practices, and children's skills. The lessons focus on three elements that supported high quality implementation and improvements in those studies: (1) a well-articulated, supplemental classroom curriculum focused on instruction in a specific domain; (2) robust professional development, including ongoing teacher training led by certified trainers and in-classroom coaching to support teachers; and (3) real-time data monitoring with data-driven technical assistance and decision-making to support teacher practices. Lessons from these studies can provide tangible guidance as administrators embark on the process of designing and implementing high-quality pre-school systems.