Most countries provide some level of support to their agricultural sectors. Because support can affect producers and consumers in other countries, a number of systems have been developed to measure agricultural support levels and classify types of support in ways that facilitate comparing them across countries. The WTO and OECD employ similar classification systems, generally addressing the same question and measuring the same programs. However, results can be surprisingly and fundamentally different, rendering comparisons inappropriate, meaningless, or even wrong. Careful attention to the sources of difference can prevent potential misunderstandings and misleading uses.