This dissertation examines how British and American people imagined and understood the transatlantic relationship in the second half of the nineteenth century. Specifically, it examines visitors' and observers' reactions to five world's fairs held between 1851 and 1893 in both nations to illustrate how these conceptions changed over the period. These events, unlike any others in the period projected national images created by government officials and industrialists in each country. They forced visitors and observers to renegotiate their understanding of the world through these curated presentations of foreign products and art. The dissertation employs newspaper reports, guidebooks, organisational documents, and other texts from these events which record visitors' changing conceptions of British and American industry, global power, and the two nations' cultural and historical bonds. These sources also record reactions from people who observed these fairs from afar without visiting them. By the end of the period British visitors and observers of these events saw the United States as part of an extended British world, bound by a shared "Anglo-Saxon" racial heritage. Americans initially highlighted their transatlantic connections at these fairs and used them to vindicate their displays. However, as American trade expanded, and its diplomatic clout blossomed in the century's final decades, American displays became more confident and robust, and U.S. visitors and observers desire for British vindication, and need to stress transatlantic connections dissipated. At these fairs British attendees became more firmly convinced of the two nations deep connections while Americans rejected any special transatlantic link by 1893. This study suggests that Americans, as their nation expanded abroad, became more firmly convinced of their national power, and did not see themselves as more similar to empires like Britain. Simultaneously British visitors and observers sought to co-opt the rising United States as an extension of themselves.