Until the late 1990s, attending a jam band concert was like going to McDonald’s. No matter where you were or who was running the franchise, you knew what to expect: hippies, weed, beads, and extended versions of songs based on American roots styles. That was comforting to millions of fans, even nurturing, so they built a loose community — let’s call it Jam Nation — around the joy they shared in that music and the gatherings it created.

Bands like Blind Melon, Blues Traveler, and the Northeast’s Max Creek made good careers by catering to the extended, idealistic vision of the ’60s that the fans who made up Jam Nation tried to live out, but none were rewarded as dearly as the Grateful Dead. After all, the San Francisco group was the foundation upon which Jam Nation was constructed. They started in 1963 as a jug band called the Zodiacs, led by guitarist Jerry Garcia and organ/harmonica player Ron "Pigpen" McKernan. Two years and a series of LSD-laden concerts later, they had evolved into a full-blown improvisational rock outfit and adopted the name that would be synonymous with jam band music for the next 30 years.

The Grateful Dead were concrete proof that people were willing to pay for their membership in Jam Nation. Between 1990 and August 9, 1995, the Dead sold $226.4 million in concert tickets — before $50 to $100 seats became common — and their corporation earned $50 million in annual revenue. But after Garcia was found dead in his room in a Forest Knolls, California, drug treatment center on that date, the Dead ceased playing and Jam Nation changed. Its heart was gone. Garcia was one of the most beloved figures in popular music, and many of the Grateful Dead’s older fans took his death personally. They lost interest in concertgoing, and the remnants of Jam Nation splintered.

Since then, it’s never been the same. And Grateful Dead bassist Phil Lesh thinks that’s a good thing. "The whole jam band culture has really come alive and diversified immensely since Jerry’s passing," he said recently over the phone from San Francisco, where the remaining members of the band were rehearsing for their first summer tour since Garcia’s death under the name the Dead — with New England stops this Saturday and Sunday. "Maybe what we need right now is diversity and richness and strangeness."

If that’s the case, the jam band scene is at a creative zenith. The Grateful Dead clones of earlier decades have been replaced by a wealth of artists who fall beneath the jam umbrella. Some, like Widespread Panic, Ulu, and the String Cheese Incident, hew fairly close to the traditionalist Dead aesthetic. But today’s jam audience also embraces veterans like the Allman Brothers and Santana. And from there the stylistic bag of post-Dead Jam Nation seems like a bizarre cornucopia.

Rock-based singer/songwriters Dave Matthews, Ben Harper, Jack Johnson, and Moe play jam festivals like Bonnaroo and Berkfest. Jam outfits inspired by bluegrass and, very likely, Jerry Garcia’s bluegrass-oriented solo recordings, include the New Deal, Railroad Earth, and the Yonder Mountain String Band. Long-established bluegrass artists David Grisman and Béla Fleck have also been adopted by Jam Nation.

Then there’s the jazz-groove division, headed by Medeski Martin & Wood, John Scofield, Karl Denson’s Tiny Universe, and the Jacob Fred Jazz Odyssey; and the funk sector led by Galactic, Addison Groove Project, and Lettuce, which has grandfathered in Parliament/Funkadelic architect George Clinton and former James Brown hornman Maceo Parker. Spearhead, Ozomatli, and even Afro-beat pioneer Fela Kuti’s son, Femi Kuti, define Jam Nation’s world music territory. The North Mississippi All Stars and pedal-steel guitarist Robert Randolph are pioneering the blues-gospel wing.

There’s even a school of bands that blithely glide between several categories. These include the gigantically popular Phish, Berklee-educated the Slip, Rusted Root, Garage a Trois, and the Almighty Senators. And the most recent breed of jammers embraces Disco Biscuits, Sound Tribe Sector 9, Particle, and the New Deal — groups who incorporate elements of electronica and other dance music into the mix.

"Almost anything can be called a jam band nowadays," says Ryan Vangel, a talent buyer for the local branch of national concert promoter Clear Channel. "With guys like [ex-Primus frontman] Les Claypool being so warmly embraced by jam audiences, soon we may be seeing [thunderous, arty hard-rockers] Tool on a Bonnaroo Festival."

The make-up of today’s Jam Nation is equally varied. During the Grateful Dead’s reign, spin-dancing legions of fans in tie-dyed togs — actual, former, or wannabe hippies — were often a large percentage of the crowd. Now there are as many young fans in backward baseball caps as in dirty jeans and sundresses kicking hackeysacks around. Entire families make outings of large jam band concerts. And, like America itself, such shows reflect greater visible racial diversity than ever before.

"There is a huge range in age in the audience, from high school and college kids attending shows by the newer band, to 60-year-olds seeing the original bands from the ’60s and ’70s," says Vangel. "Usually bands like Phish and Dave Matthews attract a younger audience, but older acts like the Dead and the Allman Brothers draw everyone from those that saw them 30 years ago to their children. I was amazed to have seen the lawn at Tweeter Center two years ago completely filled with 18- to 20-something-year-olds" for the Allmans, he says.

Jam Nation’s wide musical tastes are certainly in part a reflection of its varied make-up, but there’s also the Phish factor. If a majority of the youthful fans who were united by the Grateful Dead’s music transferred their affection to any other band after Garcia’s death, it was Vermont’s Phish. Although Phish, which formed in 1983, recorded prolifically before beginning a two-year hiatus in 2000, they acquired their huge following through ceaseless touring, graduating from the Northeast’s clubs and theaters to become one of the country’s most consistent and successful concert attractions.

Like the Grateful Dead, Phish charmed listeners with their inclination for expanding songs through onstage improvisation and by encouraging the taping of their performances. The difference is that while the Dead were idiomatically rooted save for their famed freeform "space jams," Phish tapped any musical well at whim. So a song like Phish’s "Down With Disease" might feature a spontaneous midsection flavored by Latin jazz, bluegrass licks, a disco breakdown, or a blues guitar exploration, then segue in a version of Frank Zappa’s "I Am the Slime" or Kiss’s "Love Gun" before returning to its signature riffs.

Despite its diverse character, the new Jam Nation still has the equivalent of Garcia-era Grateful Dead shows — gatherings where all elements of its membership, both musicians and fans, congregate.

"The festivals, more than anything else, are crucial to the developing bands and to the hardcore audience," says Phil Simon, whose Greenfield-based Simon Says booking agency handles some of New England’s most prominent jam acts. "Bonnaroo is the king of the kings of these events, followed by Berkfest, Gathering of the Vibes, and the High Sierra Music Festival in California." (See sidebar.)

It’s not coincidental that the reunited Grateful Dead began their summer tour at Bonnaroo, which was held June 13 through 15 in Manchester, Tennessee. "It’s reaching out to the community that has always sustained us," says Phil Lesh. And the Dead were truly one of the last old-school jam bands on the bill, which included Sonic Youth, Ben Harper, Béla Fleck, DJ Kid Koala, Liz Phair, Robert Randolph, Widespread Panic, guitarist Leo Kottke with Phish’s Mike Gordon, and Government Mule’s Warren Haynes.

Bonnaroo, despite its rural location, is as important a national event for Jam Nation as the New Orleans Jazz and Heritage Festival is to roots music fans and the Newport Jazz Festival once was to the jazz world. It sells out with minimal effort, relying on word of mouth and postings on web sites like JamBands.com and JamBase.com, the main communications trunks of Jam Nation. Just a step behind is Berkfest, properly known as the Berkshire Mountain Music Festival, held each summer in the bucolic western Massachusetts town Great Barrington.

Berkfest, produced by Boston’s Gamelan Interactive Group, helps define New England as the third-largest regional market for jam bands, trailing behind Northern California and Colorado. Last year it drew more than 10,000 people. Gamelan founder Andrew Stahl expects higher attendance during this year’s August 15 through 17 run.

The six-year-old festival has been a solid profit-maker except for its inaugural run, when 28 inches of rain dumped on the stages and campgrounds at its Butternut ski area location. Yet Stahl insists it’s about more than money. "I really do believe that we’re put here in this world to do something positive," he says. "For me, creating Gamelan was about bringing people together for shows to try to encourage positive vibrations. Berkfest is an important part of that. I don’t want to sound too hippie-dippy about it, but that’s my truth."

Nonetheless, there’s big money to be made from Jam Nation. It’s not so much in the usual merchandise, which can sometimes double the per-show take of a club-rock band, score tens of thousands of dollars for arena pop acts, and keep revenues flowing in when groups are off the road. "Heavy rock bands sell the most merch at their shows, hands down," says Clear Channel’s Ryan Vangel. "The jammier bands tend to sell less, but they have a large variety of items for sale. Instead of a typical rock band’s CDs, T-shirts, and stickers, jam bands will have 15 to 30 items, ranging from branded rolling papers to lighters and numerous shirts."

And CDs are more a vehicle for launching tours than making cash. "Jam audiences eat up live music," says Ted Kartzman, president of JamBase and comanager of the Slip, who transitioned from New England artists to national-band status following a deal with the Rykodisc label. "Albums have allowed the Slip to tour more and play more, and they’ve become more accomplished improvisers. They’ve gotten heavy into jazz and rock songwriting. Now they take 10 guitars on stage and African drums and synth drums. They get into trance music. And the audiences can’t get enough."

So, as it was in the commercial heyday of the Grateful Dead, live performance fees are the bread and butter of the jam band industry. And for major groups it’s sliced thick and spread deep. For example, the reunited Phish are playing August 2 and 3 at Loring Air Force Base in Limestone, Maine. The site is essentially an immense airfield with a campground, and fans stay for both days. Tickets, including a camping fee, are $137.50. In the past Phish have easily drawn 80,000 fans to similar locations. That’s a whopping $11 million take at the gate. If the Dead sell out their Sunday show at the 19,900-capacity Tweeter Center in Mansfield, where tickets are priced at $50 before add-on fees, the door will bring in $995,000.

That, of course, is the pinnacle of the jam band business — the province of only a few. In New England, top national draws include Phish, the Dead, the Dave Matthews Band, and the Allman Brothers, according to Clear Channel’s Vangel. The majority of jam bands are hard-touring national club and small theater acts, and regional outfits competing for much smaller grosses. National artists who do well in this region in large clubs and concert halls like the Orpheum Theatre and Avalon include the String Cheese Incident, Medeski Martin & Wood, Moe, and Widespread Panic. And Robert Randolph’s star is on the rise. He sold out two nights at the Paradise club last fall even before recording his debut studio album, which comes out on August 5.

Ulu, Jiggle, Psychedelic Breakfast, Uncle Sammy, and Hazie Mazie are among Jam Nation’s more prominent local heroes. Each can fill a Boston area club, and a two- or three-band package bill can sell out a venue like the Somerville Theater, where Stahl’s Gamelan Productions has staged many shows.

If Jam Nation has a home club in Boston, it is Harpers Ferry in Allston. Booking agent Dan Millen has cultivated that reputation, bringing in international artists like Cyro Baptista, who plays in Phish frontman Trey Anastasio’s solo group, emerging national headliners like Spooky Daily Pride and Mofro, and rising locals like Hazie Mazie. When he embarked on a jam-infused booking policy, Jam Nation was at its post-Dead high. But now, he and others involved with the music have seen some tapering off in business.

"There’s a certain peak that every style reaches," Millen observes, "and the jam genre may have reached it. There are pioneers like Phish and the Grateful Dead who come out with high-quality music, and then everybody copies them. Now the copycats are not getting the free ride that they used to. People are getting a little burnt out and a little more discerning."

When Garcia was alive and Phish in ascent, the jam-band concert business was often called recession-proof. There was a belief that Jam Nation’s sense of community would see its entertainers through hard times. But several years of a flat-lining economy and deepening unemployment does have a way of making even rabid fans more "discerning."

"We’re seeing bands that are trying to push themselves up to the $14-to-$20-per-ticket range not doing as well," says JamBase’s Kartzman. "The bands that are playing pubs on Wednesday nights for a $5 cover — they’re the ones who are consistently going to work."

Nonetheless, with such diversity in all its ranks, it’s guaranteed that Jam Nation will never go the way of the zoot-suiters and the mods. And this summer may prove to be especially important for the music. The combined buzz of summer tours by the Dead and Phish, Jam Nation’s leading groups, may be enough to open the ears of even more listeners.

"We’ll just have to see what happens," observes Lesh. "People have taken cues from us and from Phish in the past. They need something good and positive to look forward to. That’s part of human nature. So regardless of what happens, I’m proud of the spirit of unity that we represent, and I’m glad we’re still out there embodying that spirit."