Research exploring factors related to teachers' implementation of classroom-based physical activity (CBPA) has increased in recent years. This article highlights key findings from the literature about the implementation and maintenance of CBPA by classroom teachers. The authors provide an overview of the literature examining CBPA implementation from an implementation science perspective. Topics addressed include methods for measuring implementation, characteristics of CBPA resources, and predictors of teachers' implementation of CBPA. Findings from the narrative review are presented in tandem with insights from the authors' work during a grant-funded project examining strategies to improve CBPA implementation in elementary schools. Many off-the-shelf resources for CBPA implementation exist, with varying degrees of implementation support. Studies have used a variety of methods to measure implementation, mostly focused on teacher self-report. Few studies have provided evidence regarding long-term use of CBPA, but those that do report decreased implementation rates over time. A robust body of research examining CBPA implementation facilitators and barriers shows that organizational-level support from district and school administrators is crucial for successful implementation. Leadership buy-in is crucial for the implementation of policies and practices in schools, such as CBPA initiatives. Approaches that are embedded within school systems, such as communities of practice and coaching from school champions, have potential for improving implementation. Disseminating CBPA resources and providing information about the importance of school and district leadership are suggested for more widespread implementation. Future work would benefit from the use of evidence-based implementation science frameworks to ensure that interventions are designed to maximize implementation at the classroom level.