Australia and the European Union confront similar problems in seeking to ensure economic mobility within a non-unitary system of government. Both systems were established largely for the purpose of fostering interstate trade and of building a common market, but both systems also preserved much of the legislative independence of the existing States and conferred a limited legislative power on the central government. In each system it falls to its central court to judge the compatibility of state laws with the constitutional provisions aimed at achieving interstate economic mobility. The paper examines how both in Australia and in the EU the question of the scope of these provisions has seen the contest between two forms of analysis, the first form more narrowly based on discrimination against the interstate trader as compared to the intrastate trader, the second form more broadly based on the restriction of interstate trade as such. The paper also examines how the breadth of the definition of a ‘legitimate purpose’ differs between Australia and the EU, and how the ‘proportionality’ test in justification is applied with a small but very constitutionally significant difference in the two systems.