What is ideology? How can we discern significant, enduring ideas from more fleeting ones? With these opening questions the chapter lays out some ways scholars might investigate the impact of ideology on international history. The chapter offers how-to insights for historians to examine worldviews, national visions, and personal biases as they have shaped US foreign relations. In so doing, we are reminded to always consider our own ideologies, preconceptions, and assumptions, regardless of whether those presuppositions are more or less obvious. The chapter singles out key contested concepts – such as “civilization” and empire – and suggests a focus on language and rhetoric in approaching this subject. Biography and a concentration on people and groups is crucial to any deep investigation of ideology. The cultural embeddedness and historical context of the actors and ideas we focus on is critical to this work. International and transnational dimensions of thought are virtually omnipresent in the historical record; so, too, one must keep in mind the shaping role of markets and economic ideas and the impact of competing forms of nationalism. Overall, the chapter emphasizes the relationship between norms and ideology, the significance of religion, along with themes such as power, progress, and democracy.