Over the past two decades, national political and civil discourse in the United States has been characterized by "Truth Decay," defined as a set of four interrelated trends: an increasing disagreement about facts and analytical interpretations of facts and data; a blurring of the line between opinion and fact; an increase in the relative volume, and resulting influence, of opinion and personal experience over fact; and lowered trust in formerly respected sources of factual information. These trends have many causes, but this report focuses on four: characteristics of human cognitive processing, such as cognitive bias; changes in the information system, including social media and the 24-hour news cycle; competing demands on the education system that diminish time spent on media literacy and critical thinking; and polarization, both political and demographic. The most damaging consequences of Truth Decay include the erosion of civil discourse, political paralysis, alienation and disengagement of individuals from political and civic institutions, and uncertainty over national policy. This report explores the causes and consequences of Truth Decay and how they are interrelated, and examines past eras of U.S. history to identify evidence of Truth Decay's four trends and observe similarities with and differences from the current period. It also outlines a research agenda, a strategy for investigating the causes of Truth Decay and determining what can be done to address its causes and consequences.