The announcement Tuesday by House Speaker Nancy Pelosi that the House “is moving forward with an official impeachment inquiry” against President Donald Trump because she believes his actions “have seriously violated the Constitution” kicks off a complex and enormously consequential process.

But what exactly is impeachment? And how hard would it be to impeach the president and actually remove him from office?

The average American understandably isn’t an expert on impeachment. Only two presidents have been impeached by the House—Andrew Johnson in 1868 and Bill Clinton in 1999. But neither man lost his job.

Impeachment is complicated and takes time. Parliamentary democracies can quickly remove a prime minister when a majority of lawmakers cast a vote of no-confidence in the leader. But in the U.S., the impeachment process is a much tougher task to accomplish.

Here’s a Q&A on how impeachment would work.

What Is Impeachment?

Impeachment has nothing to do with the criminal prosecutions carried out by the U.S. Justice Department for violations of federal law, although such criminal violations may form a basis for impeachment.

Instead, as outlined in The Heritage Foundation’s “Guide to the Constitution,” impeachment is the process set out in Article II, Section 4 of the Constitution for Congress to remove from office the president, vice president, and “all civil Officers of the United States” for “Treason, Bribery, or other high Crimes and Misdemeanors.”

There is also a second process that applies only to the president. The 25th Amendment provides for the temporary transfer of the powers of the presidency to the vice president if a president is unable to discharge the duties of his office, such as due to a physical or other disability.

Under Article I, Section 2 of the Constitution, the House of Representatives has the “sole Power of Impeachment.”

In other words, only the House can pass a resolution of impeachment alleging that a president has committed “high Crimes and misdemeanors.” Such a resolution, which requires only a simple majority vote, is similar to a criminal indictment by a grand jury—it is an unproven list of charges that a president has engaged in actions that warrant his impeachment.

If the House passes such an impeachment resolution, then the process moves to the Senate. Under Article I, Section 3 of the Constitution, the Senate has the “sole Power to try all impeachments.”

The Senate, in essence, becomes a trial court with all of the senators sitting as the judge and jury. Based on historic practice, members of the House can act as prosecutors.

It is important to note, however, that it is entirely up to the Senate to decide whether to hold a trial. There is no obligation under the Constitution to do so.

This means that even if the Democratic majority in the House votes to impeach Trump, the Republican majority in the Senate could decide to not even consider removing him from office.

House Democrats opposed to impeaching Trump say there is no point in passing an impeachment resolution because it would most likely be dead on arrival in the Senate.

How Does an Impeachment Trial Work?

If the Senate decides to hold an impeachment trial, the Constitution says the chief justice of the Supreme Court shall preside over the proceeding. It takes a vote of “two-thirds of the Members present” in the Senate to convict any federal officer subject to an impeachment charge, including the president.

The two-thirds vote to convict means that 67 votes are needed in the 100-member Senate to remove the president and other federal officers from office. That is a very high hurdle that’s probably impossible to leap over in the case of Trump.

Democrats and independents allied with them hold only 47 seats in the Senate—meaning that even if they all voted to convict Trump, they would also need the votes of 20 Republican senators. Not a single GOP senator has called for Trump to be impeached so far, and the chances of 20 jumping on board the impeachment bandwagon are slim.

If a federal officer is convicted by the Senate, it is not a criminal conviction. The Constitution states that impeachment “shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States.”

In other words, a federal official can be removed from office. He or she can also be banned from holding any other federal office in the future.

What Happens When a President or Other Official Is Removed?

On the other hand, conviction does not bar the removed official from being “liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.”

So a federal official who is impeached, convicted, and removed from office—such as a federal judge or the president of the United States—can then be criminally prosecuted if he has violated a federal law, such as accepting bribes or engaging in treason.

How Is Impeachment Different From a Trial in Court?

The most important point to understand about impeachment is that it is not a legal proceeding like a federal criminal prosecution. And none of the formal procedural or evidentiary rules that apply to both criminal and civil trials in the federal courts are applicable in an impeachment trial.

Other than the constitutional division of labor between the House and Senate, the directive that the chief justice presides when it is the president being impeached, and the requirement of a two-thirds vote to convict, it is entirely up to the House and Senate to set the rules for how to proceed with impeachment.

It is also entirely up to Congress to determine what it considers “Treason, Bribery, or other high Crimes and Misdemeanors” that constitute grounds for impeachment.

The Supreme Court—in a 1993 case called Nixon v. United States (a case involving a federal judge named Nixon, not former President Richard Nixon)—held that the impeachment process is a political question. It is not an issue that is reviewable by, or within the jurisdiction of, the federal courts.

How Has Impeachment Been Used in the Past?

During the course of our history, the House of Representatives has impeached 19 federal officials: 15 judges (including Associate Justice of the Supreme Court Samuel Chase), one Cabinet member, one U.S. senator, and Presidents Andrew Johnson and Bill Clinton, according to a 2015 report by the Congressional Research Service.

Many people mistakenly believe that Nixon was impeached. In fact, Nixon resigned in 1974 after the House Judiciary Committee recommended impeachment, but before a resolution of impeachment could be voted on by the House.

Both Johnson and Clinton were acquitted in their impeachment trials held in the Senate.

Of the 14 other impeachment trials held, only eight resulted in convictions (all of federal judges). The last such trial (which I attended in a Senate hearing room) was of former federal Judge G. Thomas Porteous Jr.

Porteous was convicted in 2010 by the Senate on four articles of impeachment, including receiving cash and favors from lawyers who were practicing before him and lying to the FBI and the Senate during his nomination process.

What Is an Impeachable Offense?

Impeachment is probably not limited to criminal acts.

Treason and bribery are clearly criminal violations, but the Constitution does not define “high Crimes and Misdemeanors.” Alexander Hamilton argued in Federalist 65 that impeachable offenses would include “the misconduct of public men” or the “abuse or violation of some public trust.”

According to the 2015 Congressional Research Service report, both houses of Congress have in the past “given the phrase ‘high Crimes and Misdemeanors’ a broad reading, ‘finding that impeachable offenses need not be limited to criminal conduct.’”

Is Impeachment Really About the Law or About Politics?

Impeachment is primarily a political process.

If a majority of Americans do not believe that the impeachment of a president is warranted because no actual wrongdoing has occurred (or the public believes that the alleged wrongdoing is not sufficiently serious to warrant removal from office), there seems little doubt that members of Congress pushing impeachment will be unsuccessful and may suffer damaging political consequences at the ballot box.

After Republicans tried and failed to remove Clinton through the impeachment process, they lost seats in Congress in the next election. Democratic opponents of impeaching Trump fear this could happen to them if they impeach him.

The impeachment process was not placed in the Constitution so it could be used for crass, partisan gamesmanship, but was instead created to remedy serious misbehavior by federal officials.

If members of the House and Senate start voting to impeach a president because they simply oppose his policies, we could see a lot more attempts to impeach presidents in the future.

Members of Congress should be wary of abusing the impeachment authority in such a manner, because it could imperil the stability of our constitutional structure by removing a duly elected president.

Whether you are a Republican or Democrat, and whether you support or oppose Trump, you should oppose making impeachment a frequently used move against presidents of the United States. Someday, a president you think is doing a great job could be targeted.