When GRAMMY winner Gary Clark Jr. taped his "Austin City Limits" episode with Alabama Shakes in late 2012, the blues guitarist had already performed at the White House with Buddy Guy, Mick Jagger, Jeff Beck, and B.B. King, and played alongside Eric Clapton. In fact, Clark had also already played on "Austin City Limits," appearing five seasons earlier in a tribute to Jimmy Reed. But when he stood center stage before its iconic Austin skyline backdrop, finally joining a fraternity populated by so many of his idols, the Austin, Texas, native, then 28, said, "I've been wanting to do this for 16 years."

Like Clark, who learned to play guitar by wearing out his VHS tape of the 1996 tribute episode to Stevie Ray Vaughan, GRAMMY nominee Sarah Jarosz also grew up watching the public television series — which, unlike other TV programs, showcases artists performing actual, uninterrupted sets. Multi-instrumentalist and singer/songwriter Jarosz, who was raised in Wimberley, just outside of Austin, recalls, "Just getting to see some of my musical heroes on that show was pretty priceless, and inspiring."

The PBS series — the longest-running televised music program in the United States — has helped launch careers for 39 years. Even those who gain international renown before setting foot on the show's storied stage count their performance as a bucket list/holy grail moment. On Oct. 3, the night before season 40 kicks off with GRAMMY winner Beck, PBS will air "Austin City Limits Celebrates 40 Years," a two-hour special featuring some of the series' most beloved artists and rising stars, from Kris Kristofferson, Bonnie Raitt and Joe Ely to Clark and Alabama Shakes.

Co-hosted by actor/musician Jeff Bridges and GRAMMY winner Sheryl Crow, the special combines footage from a four-hour event taped in June and the show's inaugural Austin City Limits Hall of Fame induction ceremony, held in April. Among the first class of inductees were GRAMMY-winning pedal steel player Lloyd Maines, Vaughan and Willie Nelson, who taped the pilot episode of "Austin City Limits" in 1974.

ACL, as it's commonly called, has featured more than 800 live performances since it first aired 40 years ago. Conceived by KLRU-TV (then KLRN) program director Bill Arhos, producer Paul Bosner and director Bruce Scafe, the series initially focused on the unique music scene that had sprouted in central Texas, where country, blues, folk, and rock had cross-pollinated into something labeled progressive country, or "redneck rock." (Nelson's strain was dubbed "outlaw country." The nascent genre would become known as alt-country before morphing into Americana.) Broadcast during a 1975 PBS pledge drive, the show's fundraising success got it picked up for the 1976 season.

Since then, it's managed not only to stay on the air, but gain popularity, weathering the birth of MTV and other competition for viewers' attention. Time magazine has cited ACL as one of the 10 most influential music programs in TV history. It's also the only television program ever awarded a National Medal of Arts. Other accolades include a Peabody Award and its designation as an official rock and roll landmark (both the show and its venues) by the Rock and Roll Hall of Fame and Museum.

"Nobody ever thought it would go this long," says KLRU CEO and General Manager Bill Stotesbery. "Shows don't run this long in television. I think it's due to the fact that the show's maintained a very high level of quality and that it's on PBS, because PBS has a commitment to this kind of programming."

ACL also has grown in scope, reaching far beyond genre or geographical borders to feature artists from Juanes, Mos Def and Radiohead, to Buddy Guy and John Mayer, Femi Kuti and Coldplay.

ACL also grew well beyond KLRU's Studio 6A on the University of Texas campus, its home for 36 years. In 2002 the series spawned the now-annual Austin City Limits Festival and in 2011 PBS first aired the "ACL Presents: Americana Music Festival," featuring highlights from the Americana Honors & Awards Show. Also in 2011, the series relocated to the newly built ACL Live at the Moody Theater in downtown Austin on Willie Nelson Avenue, with audience capacity increasing from 350 to 2,750.

For Jarosz and performers such as GRAMMY-winning jazz artist Esperanza Spalding, who wasn't allowed to watch non-PBS programming as a child, both stages hold magic — as does the experience of playing on them.

"To get to really do a [full] set, it's like performing one act of a play or performing three acts of a play," says Spalding. "It makes a difference for the audience to see a fuller palette of what you are as an artist. … It's really such a luxury to get to stretch out and show your whole self. Forty years is a testament to that. People want to know what the artist is saying."

There's a definite career bump, too.

"Probably 90 percent of the people who come up to me after my live shows say that [ACL's] how they heard about me and that's where they first saw my performance and heard my music," says Jarosz, who was 18 when she recorded her first episode. "To be given that chance really has helped a lot."

ACL executive producer and host Terry Lickona, who also co-produces the GRAMMY Awards and is a former Recording Academy Chair, says the show's longevity has made it even more desirable to artists.

"They see 'Austin City Limits' as a validation of their music," notes Lickona, who joined ACL in its second season.  

His wish list still includes Bruce Springsteen and Prince, who's apparently a fan.

"I've heard from other people [Prince] saw Esperanza Spalding and Grupo Fantasma, and he loves to tune in to see if there's somebody new he's never heard of before," says Lickona.

Speaking of career bumps, Prince subsequently hired Grupo Fantasma as his backing band for various high-profile gigs, including a Golden Globes after-party.

Lickona also still gets a thrill from discovering new, original talent, too, and sharing it with viewers — via TV, the Internet or some other still-to-come technology.

"We're all proud of where 'Austin City Limits' has come from," says Ed Bailey, vice president of brand development for ACL. "But we're proud of where it's going. Forty years is a stopping off point to celebrate where we've been … but we're also looking for the next thing. That's why ACL still matters."

(Austin, Texas-based writer/editor Lynne Margolis has contributed to a variety of print, broadcast and online media, including American Songwriter and Paste magazines, Rollingstone.com, the Christian Science Monitor and NPR. She also writes bios for new and established artists.)