This collaborative report summarizes what the High School Tiered Interventions Initiative (HSTII) has learned about effective implementation of MTSS/RTI in high schools. It provides a brief description of the MTSS/RTI framework and the essential components of MTSS/RTI, illustrates how the essential components of MTSS/RTI are implemented at eight high schools, and highlights contextual factors unique to high schools as well as how these factors can affect school-level implementation of tiered interventions. In addition to this report, a planning tool is provided to help facilitate discussions about high school tiered interventions with school staff. This tool has been updated (June 2021) to reflect an ongoing understanding of MTSS implementation at the high school level.