David Letterman announced during the Thursday taping of his "Late Show" that he will be retiring in 2015.

R.E.M. bassist Mike Mills, who is performing on tonight's show, broke the news on Twitter.

Dave just announced his retirement #2015 #muchlovedave — Mike Mills (@m_millsey) April 3, 2014

Letterman, 66, has been a fixture on late-night television since the 1982 debut of "Late Night With David Letterman" on NBC.

When Jay Leno took Johnny Carson's "Tonight Show" host spot after he retired in 1992, Letterman left NBC for CBS' "Late Show."

From then on, it was a constant ratings battle between Letterman and Leno, ending just this February when Leno retired.

Letterman is the longest-serving late-night host in TV history. Last year he surpassed Carson at the 31-year mark. Leno had a solid 22-year run.

Craig Ferguson now reportedly has right of first refusal for a post-Letterman "Late Night" in his contract.

Here's how The New York Times' Bill Carter described it:

[Ferguson's] previous contracts with CBS have included what amounts to a "Prince of Wales" clause, giving Mr. Ferguson the right to inherit the late-night show in the 11:35 p.m. time period should Mr. Letterman decide to leave. (Those clauses have never been ironclad, however, because a network can choose to pay off the deal rather than complete the succession, as Mr. Letterman learned when Mr. Leno was chosen to succeed Mr. Carson in 1992.)

"Ferguson's CBS contract also expires in 2015, so if CBS opts to buy him out instead of giving him the prestigious 'Late Show,' he could end up leaving the network altogether, leaving the network's 12:35 slot vacant, as well," reports Splisider.

But Nikki Finke tweeted that her sources say Stephen Colbert is "the only one on the air currently that CBS is considering."

Letterman's representative released this official statement.

David Letterman, during a taping of tonight’s Late Show, said that he informed Leslie Moonves, President and CEO of CBS Corporation, that he will step down as the host of the show in 2015, which is when his current contract expires. “The man who owns this network, Leslie Moonves, he and I have had a relationship for years and years and years, and we have had this conversation in the past, and we agreed that we would work together on this circumstance and the timing of this circumstance. And I phoned him just before the program, and I said ‘Leslie, it’s been great, you’ve been great, and the network has been great, but I’m retiring,’” said Letterman. “I just want to reiterate my thanks for the support from the network, all of the people who have worked here, all of the people in the theater, all the people on the staff, everybody at home, thank you very much. What this means now, is that Paul and I can be married.” “We don’t have the timetable for this precisely down – I think it will be at least a year or so, but sometime in the not too distant future, 2015 for the love of God, in fact, Paul and I will be wrapping things up,” he added, to a standing ovation from the audience in the Ed Sullivan Theater.

Leslie Moonves, president and CEO of CBS, released the following statement:

“When Dave decided on a one-year extension for his most recent contract, we knew this day was getting closer, but that doesn’t make the moment any less poignant for us. For 21 years, David Letterman has graced our Network’s air in late night with wit, gravitas and brilliance unique in the history of our medium. During that time, Dave has given television audiences thousands of hours of comedic entertainment, the sharpest interviews in late night, and brilliant moments of candor and perspective around national events. He’s also managed to keep many celebrities, politicians and executives on their toes – including me. There is only one David Letterman. His greatness will always be remembered here, and he will certainly sit among the pantheon of this business. On a personal note, it’s been a privilege to get to know Dave and to enjoy a terrific relationship. It’s going to be tough to say goodbye. Fortunately, we won’t have to do that for another year or so. Until then, we look forward to celebrating Dave’s remarkable show and incredible talents.”

Watch Letterman announce his retirement on Thursday's show.

And so the countdown begins.