A growing wave of studies in international relations is interested in “infrastructure.” Pipelines, ports, financial transaction arrangements, and other large technical systems increasingly occupy the minds of international theorists. This theory note provides direction to the debate by offering an important clarification of the concept of infrastructure and how it is theorized. Scholars have very different understandings of what infrastructures are, why they matter, and how to theorize and study them empirically. By outlining three distinct “styles of theorizing infrastructure,” we provide new directions for future research and how it can contribute to broader debates in international theory. The three styles allow to capture the disagreement over whether infrastructure is a theoretical concept at all, or if it solely refers to empirical phenomena. For some scholars, infrastructures are an object of politics, while others see them as developing political force or even agency on their own. Others see broader potential and note that “infrastructuralism” could provide a major new theoretical vocabulary. Each style provides major new avenues for international theory.