How can governments train thousands of teachers effectively and at scale? This implementation research shares evidence from Sierra Leone's nationwide initiative to train 12,500 Grade 1 teachers and headteachers in structured pedagogy. Drawing on test data from over 7,500 teachers, classroom observations, and feedback from more than 5,600 participants, the report explores what worked, what didn't, and why. It highlights the strengths and limitations of the cascade training model, explains variations across districts and topics, and identifies ways to sustain teacher professional development beyond one-off workshops. [Additional support for this report was provided by What Works Hub for Global Education (WWHGE) and the United Kingdom Committee for UNICEF.]