There are nearly 200 million lower and upper secondary age adolescent girls and boys out of school globally, and if current trends continue, another 825 million children will not acquire basic secondary-level skills by 2030. Global momentum to achieve universal primary education has placed increased demands on secondary education systems to serve more students, from a wider range of backgrounds, and to do so more effectively and inclusively. Many of the most marginalized adolescents in lower income countries who are enrolled in school have fallen years behind their peers. In alignment with the focus on the most marginalized adolescent girls and boys brought forward in the UNICEF Strategic Plan (2018-2021) (see ED608930), this guidance offers differentiated recommendations for countries at various stages of the progressive universalization of lower and upper secondary education. It presents principles of good programming, key concepts, and useful tools and resources to orient CO programming on secondary education, using data to identify where and which adolescents are in and out of the education system and why. Additionally, it presents programming recommendations and illustrative case studies aligned to the strategic plan, as well as costing and financing models, monitoring and evaluation tools, and insights on advocacy and partnerships.