This thesis investigates ‘subsidiarity’, a principle commonly understood as a preference for authority to be exercised as close as possible to the people. Specifically, the research challenges criticisms that subsidiarity has no real meaning, both in the sense that it is difficult to conceptualise or define (i.e., what does ‘subsidiarity’ mean?), and because some argue it has little benefit or importance for society (i.e. it is meaningless). Against these accusations, the thesis presents a new, clearer way of conceptualising subsidiarity, and evidence from a survey of public attitudes conducted in Australia, Canada, Germany and the United Kingdom. First, it addresses the difficulty of pinpointing subsidiarity’s meaning in a definitional sense by drawing together various developments and approaches in thinking about subsidiarity, to propose a conceptualisation of the principle as being a product of decentralist (government at the lowest possible level), non-absorptionist (higher orders should not absorb the functions of lower orders) and supportive (higher orders should help lower orders to help themselves) approaches to governance. This new way of understanding subsidiarity provides a meaning to subsidiarity that is clear enough to be useful, but also flexible enough to accommodate the different interpretations used in academia and government. Second, it demonstrates that subsidiarity is meaningful to citizens by measuring public attitudes towards decentralist, non-absorptionist, and supportive elements of subsidiarity in Australia, Canada, Germany and the United Kingdom. Exploring public attitudes is an important departure from previous approaches, which have almost exclusively relied on analysis of constitutions, court decisions and government policy to understand the principle’s meaning and impact in society. Using insights from the way citizens of these countries value decentralism, non-absorption and support, the thesis demonstrates widespread attachment to subsidiarity, but it also reveals variations in support for these conceptions, suggesting distinct ‘subsidiarity political cultures’ consistent with the history, institutions and broader political culture of each country. For example, it shows that on average, Canadians and Germans value the principle the most, but that German subsidiarity political culture tends to value non-absorption more highly than the other elements, while Canadians are more attached to supportive subsidiarity. Meanwhile, Australians remain comparatively unattached to subsidiarity as a decentralist principle, but place great value on the idea that higher levels of government should support lower levels, opening significant new lines of inquiry for how reform in Australia could be approached in the future. These findings affirm that subsidiarity does have enduring meaning to citizens as a principle of political organisation, but also suggest that differences in subsidiarity’s meaning can and should be embraced as a guide to reforms that may better reflect citizens’ values.