This guidance note is about how donors, can support a demographic transition in sub- Saharan Africa. The demographic transition is the evolution from high to low mortality and fertility rates, with associated changes in age structures. Countries in sub-Saharan Africa are on a trajectory of rapid population growth. Mortality rates have been declining for some time while fertility rates started to fall later and at a slower pace, resulting in high population growth. It is estimated that the population of sub-Saharan Africa will double between 2020 and 2050 to 2.5 billion. This guidance note refers to support from donors to governments in partner countries in two ways. First, support to adapt to the implications of rapid population growth. Second, support to accelerate the demographic transition. Countries in sub- Saharan Africa need to be prepared for population growth and, importantly, also for a unique “window of opportunity” that occurs when fertility rates fall consistently and at a high pace during the demographic transition. With the right investments, these countries could generate economic opportunities for growth, which in the literature is called the “demographic dividend”.