A quality secondary school education is measurably associated with positive effects on productivity, well-being, and health. However, many children in developing countries are leaving primary school without basic literacy and numeracy, and those who do make it to secondary school are often unprepared for the higher levels of learning required. While primary schools are now widespread internationally with near equal numbers of boys and girls enrolled, secondary schools remain scarce, are mostly in central towns and urban areas, and enroll fewer girls than boys and too few poor and disadvantaged children. Moreover, the nature of employment in developing countries is changing from a focus on widespread subsistence agriculture and small-scale industrial sector work to a much more complex mix of formal and informal economic activities. Many questions surrounding accessibility, quality, and relevance remain unanswered. This makes targeting and calibrating the scale-up of secondary education even more challenging. Innovation is needed to ensure more marginalized young people are able to access and complete quality, relevant, secondary education, and demonstrate learning.