In most contexts, adolescents also progress from the relatively small, family-based world of childhood into a wider world of peers, colleagues and possibly new families. The years of early adolescence are found to be especially important because of the ways in which norms start to become more rigidly enforced. Noting the centrality of gender norms to the day-to-day lives, A. M. Boudet observe that they 'permeate daily life and are the basis of self-regulation'. The Girls' Opportunity Index focuses in on opportunities for girls - ranking the world's countries in terms of child marriage, adolescent fertility, lower secondary school completion rates, maternal mortality and women Members of Parliament. The conceptual framework that serves as the basis for the research draws on the 'capabilities approach' that has emerged over the past decade as a leading alternative to standard economic frameworks for thinking about human development, inequality and social justice. The chapter also presents an overview on the key concepts discussed in this book.