This framework marks the first in a series of five reports detailing the work of the Optimizing Assessment for All (OAA) project at Brookings to strengthen education systems' capacity to integrate 21st century skills (21CS) into teaching and learning, using assessment as a lever for changing classroom practices. In a world of rapid advancement and change, people and societies need a new mixture of skills to thrive. It is no longer accepted that education devoted mainly to the memorization of facts will prepare people to be thoughtful, productive, and engaged citizens. Rather, people of all ages need a broad set of skills, including cognitive, social, and technological, to evaluate and apply knowledge in ways that meet the new demands of a changing social and economic landscape. These skills, such as critical thinking, problem solving, collaboration, information literacy, and communication, among others, are often broadly referred to as 21CS or transversal competencies. Countries around the world are including 21CS in their national education goals and policies, signaling their commitment to addressing the changing needs of their learners. However, countries are faced with challenges on how to implement these policies due to disconnects between different parts of the education systems; lack of understanding on how teaching and learning 21CS looks in the classroom; and limited measurement expertise in the area of 21CS assessment. Between December 2017 and April 2020, the Optimizing Assessment for All (OAA) project will address these challenges by strengthening education systems' capacity to integrate 21CS into their teaching and learning, using assessment as a lever for changing classroom practices and building that capacity. The OAA project has emphasized developing existing capacity and resources and changing mindsets to cultivate and support a new generation of assessment specialists within the participating countries and regions. OAA has worked in two regions--Asia and sub-Saharan Africa--and three focus countries within each region have participated in a capacity-building approach for designing, developing, and piloting classroom-based assessments of 21CS. [Support was provided by Porticus.]