This report describes findings and lessons learned from an online teacher training course in the Philippines. "Becoming a Learning Detective" is a 5-day online course that focuses on the design and use of formative assessment to improve literacy and numeracy outcomes in Kindergarten to Grade 3 (K–3) classrooms. The emphasis is on classroom-based assessment strategies that are embedded within daily teaching and learning experiences, involving an active partnership between teacher and students. The course brings together asynchronous and synchronous elements, as well as whole group, small group, and individual learning experiences. The course was implemented for the first time among a small cohort of participants across 17 regions in the Philippines. Findings and recommendations of this initial training are being applied toward strengthening the course prior to offering it at a larger scale in the Philippines.