Many countries have considered extending their school days to improve students’ outcomes, promote equity or support parents to combine work and family lives. Given the impact of such reforms, identifying conditions for their successful implementation is an important concern. This working paper reviews the available evidence and synthesises common lessons from six European and Latin American countries that extended and reorganised their school days. Each case study describes the reform’s context and goals, design and implementation, and resource implications. The paper highlights that lengthening the school day might be an efficient strategy for some schools and systems, but not for others, depending on policy goals and alternatives. To reap any potential benefits, reforms need to consider the quality and articulation of the activities taking place and related adjustments to school resources. As the paper suggests, school-day extensions provide an opportunity to rethink schools as places not just for learning, but for holistic student development, engagement and support.