This introductory chapter provides an overview of Wilsonianism, which comprises a set of ideas called American liberal internationalism. More than a century after Woodrow Wilson became president of the United States, his country is still not certain how to understand the important legacy for the country's foreign policy of the tradition that bears his name. Wilsonianism remains a living ideology whose interpretation continues either to motivate, or to serve as a cover for, a broad range of American foreign policy decisions. However, if there is no consensus on what the tradition stands for, or, worse, if there is a consensus but its claims to be part of the tradition are not borne out by the history of Wilsonianism from Wilson's day until the late 1980s, then clearly a debate is in order to provide clarity and purpose to American thinking about world affairs today.