The period 1517–1625 was crucial for the development of political thought. During this time of expanding empires, religious upheaval, and social change, new ideas about the organization and purpose of human communities began to be debated. In particular, there was a concern to understand the political or civil community as bounded, limited in geographical terms and with its own particular structures, characteristics, and history. There was also a growing focus, in the wake of the Reformation, on civil or political authority as distinct from the church or religious authority. To explain these new ideas about political power, the concept of sovereignty began to be used, alongside a new language of reason of state. Yet political theories based upon religion still maintained significant traction, particularly claims for the divine right of kings. In the midst of these developments, the language of natural law became increasingly important as a means of legitimizing political power; natural law provided a rationale for earthly authority that was separate from Christianity and its use enabled new arguments for religious toleration. This book offers a new reading of early modern political thought, drawing on a wide range of sources from Europe and beyond. It makes connections between Christian Europe and the Muslim societies that lay to its south and east, showing the extent to which concerns about the legitimacy of political power were shared. It demonstrates that the history of political thought can both benefit from, and remain distinctive within, the wider field of intellectual history.