Much has been written about the growing disregard for facts, data, and analysis in political and civil discourse in the United States. Increasingly, it seems that important policy debates, both within the federal government and across the electorate, are as likely to hinge on opinion or anecdote as they are on objective facts or rigorous analysis. However, policy decisions made primarily on the basis of opinion or anecdote can have deleterious effects on American democracy and might impose significant costs on the public. The current discourse about the diminishing role of, trust in, and respect for facts, data, and analysis is often hamstrung by the use of conflicting language and unclear or undefined terms. Without a common language with which to discuss the problem—which we are calling Truth Decay—the search for solutions becomes more difficult. This report seeks to address this gap by offering a clear definition of Truth Decay and an examination of its drivers and consequences—all with the aim of creating a foundation for more-meaningful discussion of the challenges to U.S. political and civil discourse. The report outlines a research agenda designed to guide further study of Truth Decay and the formulation of responses. The report is the first of several publications that will discuss Truth Decay in different contexts and from different angles.