For teachers to nurture and guide students’ development of 21st century skills (21CS), many pieces of the education puzzle must fall into place. The Optimizing Assessment for All (OAA) project set out to focus on one of these pieces: development of assessments for use in the classroom. These assessments would inform teachers’ formative practices to nurture 21CS, as well as the design and development of assessments for use at larger scale. Larger-scale assessment in turn would inform evaluation of system progress in implementation of 21CS policies and practices. Six countries engaged in the OAA project: Cambodia, the Democratic Republic of Congo, The Gambia, Mongolia, Nepal, and Zambia. This paper marks the final installment in a series of five reports detailing the work of the Optimizing Assessment for All project at Brookings to strengthen education systems' capacity to integrate 21CS into teaching and learning, using assessment as a lever for changing classroom practices.