President-elect Trump claimed "millions of people" voted illegally in the election. There is no evidence of that. Here's everything you need to know:

How common is voter fraud?
The evidence suggests it is extremely rare. In the 2016 election, state officials found a few dozen suspected incidents of voter fraud among the 137.7 million votes cast. All previous research conducted on voter fraud has reached the same conclusion: A tiny, insignificant number of people attempt to impersonate other voters in order to cast ballots. In a comprehensive study, the investigative journalism organization News21 identified just 2,068 incidents of alleged election fraud among the hundreds of millions of ballots cast in all U.S. elections between 2000 and 2012. A 2014 study by Loyola Law School in Los Angeles unearthed only 31 instances of voter impersonation among the more than 1 billion ballots cast in all U.S. elections since 2000. Richard Hasen, an election-law expert at the University of California, Irvine, says that data from this and previous elections show that Trump's claim of millions of illegal votes is "obscenely ludicrous."

So why is voter fraud a hot-button issue?
The potential for fraud certainly does exist, and many Republicans contend that the number of voters caught casting illegal ballots may be just the tip of the iceberg. A 2012 Pew Center report found that one in eight voter registrations was inaccurate, out of date, or a duplicate; that 2.8 million voters were registered in more than one state; and that 1.8 million dead people remain on the electoral rolls. In 2013, undercover agents from New York City's Department of Investigations assumed the names of people who were no longer eligible to vote because they had died, left town, or were in jail, and 61 of the 63 polling places they visited allowed them to vote. Since only 31 states require voters to show some form of ID at the ballot box, Republicans say, it's possible that illegal immigrants, convicted felons, and politically motivated frauds are casting ballots without being detected.

Has that ever been investigated?
Yes, and the results do not support widespread voter fraud. High-profile "purges" in Florida and Colorado in 2011 and 2012 found only a few hundred illegally registered voters out of 15 million. After Kansas Secretary of State Kris Kobach in 2015 asked for prosecutorial powers to go after voter fraud, he filed charges in just six cases, two of which were thrown out. When the Justice Department carried out a crackdown on voter fraud under President George W. Bush, it yielded 86 convictions nationally — many of which were the result of simple human error. "If you were here as an undocumented person, or even someone who has a green card," says Lori Edwards, an elections supervisor in Florida, "why would you risk that status for what would be a minimal benefit?" Election officials also argue that for fraud to have any chance of influencing state or national elections, tens of thousands of people would have to go to polling places pretending to be someone else — a massive operation to organize and carry out without anyone ever finding out. Supporters of voter ID laws, however, argue that even one fraudulent vote is one too many — and that stricter voting laws are the only solution. "It doesn't matter if there's one, 100, or 1,000," says Wisconsin Gov. Scott Walker. "Who would like to have [their] vote canceled out by a vote that was cast illegally?"

Do Democrats disagree?
Yes. They contend that Republicans exaggerate the effect of voter fraud in order to pass laws making it more difficult for blacks, Hispanics, and the poor to vote. No state required any form of voter ID until 2008, when Barack Obama won the presidency. In 2013, the U.S. Supreme Court ruled that states with a history of racial discrimination no longer had to acquire federal approval to change their election laws, and 14 Republican-controlled state legislatures quickly implemented new restrictions on voter ID, early voting, and same-day voter registration — all of which disproportionately affect minority voters. About 13 percent of African-Americans and 10 percent of Hispanics, for example, lack valid photo ID. A federal appeals court ruled last year that North Carolina's voter law targeted African-Americans "with almost surgical precision." Before passing the law, Republican legislators specifically asked elections officials for data about black registration patterns and what percentage of black voters lacked photo ID.

Do these laws depress turnout?
The evidence on that is mixed. When the nonpartisan Government Accountability Office looked into the issue in 2014, five of the 10 studies they examined showed that ID laws had "no statistically significant effect on turnout." But a recent University of California, San Diego, study concluded that "Democratic turnout drops by an estimated 8.8 percentage points in general elections when strict photo identification laws are in place" — enough to swing close elections. Whether or not that's true, both parties apparently believe that it is. Republican political consultant Carter Wrenn, a longtime fixture in North Carolina politics, admitted this year that the state's voter ID law and attempts to restrict early voting were designed to suppress Democratic votes. "Of course it's political," Wrenn said. "Why else would you do it? Look, if African-Americans voted overwhelmingly Republican, they would have kept early voting right where it was."

Voting laws under Trump
The Trump administration will very likely reverse the Obama administration's fierce opposition to voter ID laws, and throw its legal support behind states that seek proof of identity at the ballot box and a limit on early voting. Trump will also probably appoint a Supreme Court justice who will side with states that impose voting restrictions. President-elect Trump's nominee for attorney general, Alabama Sen. Jeff Sessions, has called the Voting Rights Act "a piece of intrusive legislation," and was denied a federal judgeship in 1986 in part because of his attempts to prosecute three activists who helped elderly black voters cast absentee ballots. In 2013, Sessions praised the Supreme Court's ruling to weaken the Voting Rights Act as a victory for the South. "If you go to Alabama, Georgia, North Carolina," Sessions said, "people aren't being denied the vote because of the color of their skin."