David Letterman.

David Letterman was a breath of fresh, irreverent air when he first arrived on late-night TV.

Hard as it is to believe, that was a full quarter-century ago.

Tonight's episode of Late Show With David Letterman (CBS, OMNI1, 11:35 p.m.) marks Letterman's 25th anniversary as a late-night host.

And for the occasion, Letterman has invited back Bill Murray, who was the first guest on the debut episode of NBC's Late Night With David Letterman on Feb. 1, 1982. There was a time when the terms "top 10 list" and "stupid pet tricks" were not part of pop culture in North America. But Letterman has displayed a longevity that few would have predicted when they got their first look at him.

"Check out the gap-toothed smart-ass," audiences undoubtedly said. "Who does he think he is, the next Johnny Carson?"

As it turned out, it was NBC's unwillingness to hand Letterman the time slot occupied by Carson upon the latter's retirement that caused Letterman to jump to CBS. Letterman has been battling for ratings with Jay Leno ever since.

Murray also returned for Letterman's CBS debut on Aug. 30, 1993, but that particular show was a tad disappointing, to be honest. Back in 1982 a frenzied Murray did jumping-jacks while singing Olivia Newton-John's Physical, and with the pressure on to be equally zany 11 years later, Murray couldn't come up with anything even remotely as memorable.

Letterman is 59 now and Murray is 56, so the chances of them doing something "physical" tonight are fairly remote. But there should be some good stories amid the nostalgic atmosphere.

Also on the show tonight is NBA star LeBron James, who, coincidentally, is only 22 years old. Chances are he won't be joining Letterman and Murray in a discussion about the good ol' days.

"It has been a long run," Letterman's right-hand man and musical director Paul Shaffer said in a recent interview with Sun Media. "It's bound to go up and down. But right now, we're in a phase where we're having an awful lot of fun.

"The thing is, David Letterman is so quick and unpredictable. The show is unrehearsed. It really is just flying by the seat of your pants, and that's what makes it fun on a daily basis.

"Really, I still look forward to doing it each day. It truly is a blessing, and I don't take it for granted, either. I'm aware of how rare it is for a musician to have a job that lasts so long -- and such a high-profile one, too."

All told, Letterman has amassed 4,506 broadcasts, 14,772 guest appearances, 14 Emmy Awards and 89 Emmy Award nominations.