There is a cross-national trend towards unified curriculum and qualifications frameworks in upper secondary education, but such reforms face epistemological, political and institutional barriers and "unification" remains a contested issue in many countries, including England. This article examines the experience of the Scottish Higher Still reform, one of the most systematic examples of this trend. It presents data from an Economic and Social Research Council-funded study which included case studies of schools and colleges, surveys of all secondary schools and colleges in Scotland, analyses of Scottish Qualifications Authority (SQA) data and interviews with key informants. Higher Still's climbing frame model of provision provided better learning opportunities for different types and levels of students, but its impact on attainment and parity of esteem were more limited. The article concludes that institutional barriers formed the biggest obstacles in the implementation phase, reflected in the contrasting "institutional logics" of schools and colleges. (Contains 13 tables.)