In the mid-sixteenth century, there were many parallels between the political cultures of Poland-Lithuania and the kingdoms of the British Isles, as thinkers inspired by the ideals of Renaissance civic humanism challenged traditional currents of thought. Across the British Isles and Poland-Lithuania there were strong native traditions asserting the liberties of communities of the realm and the need to check unbridled royal authority through parliamentary assemblies. As the Reformation swept across the British Isles and Poland-Lithuania, traditional claims concerning the right to resist tyrannical authority were bolstered. Finally, in 1603, Scotland and England formed a loose political union as Poland and Lithuania had formed a loose political union in 1386, although it was not until 1707 that England and Scotland followed the example of the 1569 Lublin Union, when Poland and Lithuania established the first parliamentary union in European history. Despite these parallels, the fates of these composite polities were very different, and their political cultures diverged substantially. This article considers the idea of the Renaissance republic in Poland-Lithuania and the British Isles. It suggests why their roads diverged, and asks what made all the difference.