This paper sets out the main findings of the International Instructional Systems Study (IISS), conducted by the UCL Institute of Education and funded by the Center on International Education Benchmarking (CIEB). The study examined the instructional systems and intended curricula of six "high performing" countries and two US states. The study ultimately focused on nine specific aspects of those systems: the aims of the education system; how centralised or decentralised management of the instructional system is; principles and methods of accountability; what compulsory and optional subjects are included in the programmes of study; the degree to which curriculum is organised by discipline or integrated across disciplines; whether curriculum is common or differentiated; how twenty-first century skills are embedded in the curriculum; the clarity and content of curriculum for secondary vocational pathways; and how assessments are created and what stakes they have and for whom.