The size of the private residential sector is more than half the housing stock in some countries but less than one tenth in others. This variation might be the result of sets of social, economic and institutional factors that have operated over many years. The observed differences may also be related to definitions and the ways that data are recorded and presented. Policy measures by governments that influence the composition of the housing stock and the supply of housing in various tenures may also have an impact. It is the last set of factors that is the focus of this presentation. The paper stems from research for Department of Communities and Local Government, Promoting Investment in Private Rented Housing International Comparisons (2010); ESRC, Knowledge Exchange Project, Boosting the Supply of Affordable Housing: Learning from other Countries (2013) both led by the speaker and work at CCHPR for Realdania, Denmark: The Private Rented Sector in the New Century – A Comparative Approach, (2010-2012).