The paper concludes with a brief note on the way forward. For the OECD as a whole, the tax to GDP ratio rose from 28.6% of GDP in 1975 to 34% in 2015. The tax mix also varies, reflecting a host of determinants, including among others the structure and openness of the economy, inter-governmental arrangements (e.g. unitary versus federal structure) (Box 2), the institutional capacities of the national and sub-national government administrations, and collective attitudes toward income redistribution and the use of progressive taxation. Sub-central entities generate their own revenues, to varying degrees.