This paper addresses the following questions about immovable property taxation in OECD and partner countries: What is valued? How is it valued? And who values? It draws on published information and data on property tax policy and administration in 172 countries. It focuses on value-based taxes and the features of mass valuation systems. Main system options (such as whether taxes are based on annual rental values or capital values as reflected by sales prices) are described and briefly evaluated. It notes that valuation practices frequently ignore revaluation requirements; it identifies four areas for improving valuation performance based on the experiences of leading systems.