Agriculture’s water quality and quantity challenges continue to grow in many regions of the world. Policy solutions have been identified, but not always applied where needed nor implemented effectively. This report analyses eight past water and agriculture policy changes in OECD countries with the aim to identify steps towards adopting and implementing such solutions effectively. Selected reforms are assessed systematically via an institutional change analysis and a cross-cutting comparison of political economy factors. A characterisation of reforms is proposed according to the scope of the reform process, the scope of the reform’s action, and the involvement of governments in the design of reforms and their implementation. The comparison of agriculture and water policy changes shows that introducing reforms can be facilitated by exogenous factors, including droughts and floods, and reform design features. Meanwhile, the outcome of reforms can be affected by their geographical scale and scope, the dynamic pattern of reform pathways, and compensation for farmers. There are, however, trade-offs between the effects of these factors on the reform’s ambition, effectiveness, efficiency, and flexibility.