Comparisons are made between PISA 2012 and PISA 2003, the last time mathematics was assessed in depth. To account for the extent to which the observed relationships are influenced by the level of economic development of countries and economies, the comparison of school systems discussed in this chapter also considers national income per capita (per capita GDP). Among all countries and economies that participated in PISA 2012, 23% of the performance variation among students is observed at the system level, 31 % is observed at the school level, and 46% is observed at the student level. The cross-national analyses provide an overview of how system-level attributes and major organisational arrangements relate to student performance and equity in school systems.