This article documents the sources, character and impact of political polarization on the U.S. Congress and American politics generally. Partisan polarization among the public, activists, and elected officials characterizes American politics in the 21st century, and it is often seen as the major source of our governing problems, especially in Congress. In the 2016 election, Republicans and Democrats are further apart ideologically than at any point in recent history, expressing highly negative views of the opposing party. This deleterious impact of partisan polarization cannot be exaggerated. It discourages compromise, produces gridlock, fosters mistrust, and ultimately hinders the functioning of governmental institutions. Lawmaking, representing, overseeing, executing laws, and adjudicating legal and constitutional disputes requires that individuals who might otherwise disagree come together and serve interests that go beyond their own. Failure to compromise results in gridlock, dysfunction, and partisan warfare as we see in 2016 and likely beyond with a new Congress and President.