Spending reviews are widely used in OECD countries. They have become a core instrument for expenditure prioritisation and reallocation and a permanent feature of the budget process in many countries. Spending reviews provide governments with means to support the sustainability of public finances through systematic analysis of existing expenditure. This is particularly important as baseline expenditure has grown unsustainably in certain spending areas across OECD countries. This paper looks at how spending reviews are applied in OECD countries and presents best practices for using spending reviews. Every country is different and uses different approaches. However, there are common features to successful spending reviews where valuable lessons can be drawn from experiences across OECD countries which are reflected in the best practices.