Without Robert Klein, modern stand-up comedy might not exist.Klein was part of the holy trinity of the stand-up pantheon - along with George Carlin and Richard Pryor -  of the post-Beatles era. That rock n roll consciousness proved crucial to the changes the three comedians wrought on what had become a tired, Borscht Belt-and-vaudeville-derived art form.Klein, Carlin and Pryor were all figurative offspring of the late, pioneering Lenny Bruce, who blasted boundaries with his free-associative comedy, winding up as a free-speech martyr and improv god. Robert Klein hit the main seam, taking observational comedy to new heights by finding the humor in everyday things, whether it was a trip to the dentist or life in high school in the nuclear age. Robert Klein was talking to us: the baby-boom generation that had screamed for the Beatles, postured with the Rolling Stones, grooved to The Mod Squad.It's hard to overestimate how fresh and unexpected Klein's comedy seemed when he first appeared on television. His observational humor -  brash, intelligent, edgy without being confrontational -  provided a role model for a generation of comedians that followed: Richard Belzer, Billy Crystal, Richard Lewis, Jerry Seinfeld. His first album,  Child of the 50s,  became a kind of Bible for the comedically-inclined who were following in his footsteps.Klein came from theater: Yale Drama School, to be exact. A Bronx native who graduated high school at 16 and college at 20, he quit Yale because he thought he could make it as an actor in theater in New York. Not quite; but then he auditioned for Second City and spent 14 crucial months learning and performing in Chicago, before returning to New York and Mike Nichol's musical,  The Apple Tree.After the curtain fell on Broadway each night, Klein would rush to the Improvisation, the pioneering comedy club on West 44th Street, where he'd absorb the master class in comedy being conducted by Rodney Dangerfield. Dangerfield saw potential in the good-looking, gangly Klein and took the eager would-be comic under his wing, helping to find him management and to launch his career with a  Tonight Show  appearance in 1968.By 1975, when the first of the HBO concerts in this set aired, Klein was king, established as a comic wonder on variety and talk shows and working as an actor, while recording comedy albums. His first HBO appearance was also a first for HBO: its first live comedy concert, a show that set new benchmarks for audacity and language, demonstrating that the sparks that are struck during live performance can be felt through the medium of cable television.It's hard to think of a comic who has had the staying power and longevity that Klein has. As this boxed-set shows, he has always been a clear-eyed observer of the world around him, willing to yell  Bullshit!  in a crowded theater as much out of a sense of personal outrage as to get a laugh.More than 30 years after that first HBO special, Klein still knows how to cock a knowing eyebrow at the foibles and follies of his time. Older now, he is the parent of an adult son, but has resisted turning into his own parents. Still irreverent, still bitingly intelligent, his comedy remains fresh and telling. He was a child of the 50s - and then a child in his 50s - and now he is a citizen of the 21st Century. And still a damned funny one.Marshall FineAn Evening with Robert KleinRobert Klein RevisitedRobert Klein at YaleRobert Klein: Child of the 50 s, Man of the 80 sRobert Klein on BroadwayRobert Klein: It All Started HereRobert Klein: Child in his 50 sRobert Klein: The Amorous Busboy of Decatur AvenueBONUS EXTRA:A Conversation with Robert Kleininterviewed by Alan Colmes (2007)