Josh Radnor, guest star Carrie Underwood and Neil Patrick Harris in "How I Met Your Mother"

A special guest star is a great tactic to deliver a ratings punch for any television series. But one series that's attracting big-name celebs doesn't need any help in that department.

Country music sensation and "American Idol" winner Carrie Underwood is the latest high-profile star to grace the "How I Met Your Mother" cast. She'll appear on the show Monday, March 1, on CBS.

Underwood will play Tiffany, a gorgeous young woman who catches Ted's attention. While Ted is ga ga for Tiffany, the gang tries to convince him that she's just stringing him along, and everyone reminisces about being in a similar situation.

Underwood's been popping up on the small screen a lot lately. It's understandable since it launched her career in 2005 when she won the fourth season of "Idol." But a few months ago, she hosted her own variety special, and she's hosted the Country Music Awards for the past two years, as well as popping up on various other awards shows to perform or just pick up a little hardware.

Her job as an actress on "Mother" is certainly a new experience for her, though the plotline is a pretty typical one for the hit comedy series, which follows the journey of a young man as he recounts falling in love with the mother of his children. The story is told in flashbacks, narrated by Bob Saget as the future Ted. In the present, Ted is played by Josh Radnor.

Ted's quest is put in motion when his best friend Marshall (Jason Segel) proposes to his girlfriend Lily (Alyson Hannigan). Neil Patrick Harris, most recognizable as the one-time star of "Doogie Howser, M.D.," plays Barney, an affable scoundrel who helps Ted with women. Cobie Smulders stars as Robin, Ted's former love interest who ends up hooking up with Barney, though it seems that wasn't meant to last, either.

The twentysomething vibe of the series has attracted scores of loyal fans who tune in each week to see how the gang navigates the tricky waters of dating, romance and friendship. "How I Met Your Mother" often leads in the Monday night ratings, and the show is getting even more attention lately thanks to some glamorous guests.

An upcoming appearance from J Lo herself has viewers and music fans buzzing. In January, CBS announced fashionable actress and pop star Jennifer Lopez would appear in a March episode of the comedy, sparking a media frenzy.

Many would argue Lopez is the biggest star to hit the "How I Met Your Mother" set, but there's another frontrunner who made waves a couple of seasons ago. Pop princess Britney Spears guested on the series as a potential love interest for Ted in March 2008.

The performance, well received by most fans, marked a turning point for the young performer whose life had been in shambles following custody issues, drug abuse allegations and bizarre behavior. It was Spears's first legitimate gig since much of the trouble began, and she proved herself a professional by stepping up to the role as a receptionist who takes a shining to Ted.

It isn't just red-hot music stars making guest appearances on the show. Regis Philbin, the king of a.m. talk shows, guest starred as himself last season. Comedian and actor Alan Thicke has also appeared on the show, as well as Michael Gross, James van der Beek and '80s pop star Tiffany.  

For a series that consistently attracts more than nine million viewers per episode, such guest stars are just the icing on the ratings cake. With a strong ensemble cast that has garnered praise from both viewers and industry insiders, it appears "How I Met Your Mother" can do no wrong lately.

The cast even pulled off an all-out musical episode earlier this season, complete with cheesy songs and Broadway-style choreography. While all of the series stars held their own, it was Harris who really shone in the episode, confirming what fans have known for ages: that Barney rocks.

Although Radnor (who recently snagged the audience award in the dramatic category at the Sundance Film Festival for his work on "Happythankyoumoreplease") is no slouch, Harris has always been the standout on "How I Met Your Mother." It was an uphill climb for the one-time child star, who many worried wouldn't be taken seriously as an adult.

The actor has proved his acting mettle with big-screen roles, as well as roles on Broadway. He's also hosted both the Tony and Emmy awards.

Regardless of his outside activities, Harris's "Barney" has viewers hooked, and CBS certainly has faith in the entire "How I Met Your Mother" cast and crew. The show has already been renewed for a sixth season, an early pickup considering that at one time, early on in its run, "How I Met Your Mother" fans had to wait until spring to find out whether the show would be back.

Also of note is the fact that series co-creators Carter Bays and Craig Thomas have signed a deal with 20th Century Fox to create a new show.