Heidi Tandy still remembers the dread she felt the day in 2002 when she received an official-looking email from Warner Bros., the studio that produced the Harry Potter films. Tandy was one of the founders of FictionAlley, a website dedicated to Potter fan fiction. She’d first become involved in fan culture back in 1994, on an email listserv for Friends buffs. Like everyone she knew, she assumed that any communication from rights holders (the Powers That Be, or TPTB in fandom lingo) meant a serious danger to her site—a cease-and-desist notice, or even a threat to sue.

However, as Tandy recalls in a collection of essays edited by Anne Jamison, Fic: Why Fanfiction Is Taking Over the World, it turned out that the studio’s representative had seen a front-page article about Pottermania in the New York Times and wanted to suggest that FictionAlley become an associate of the Warner Bros. store. Although this sounds like a perfectly mundane exchange today, that studio email marked a watershed. Fandom in all its forms, thanks in large part to Harry Potter, had become legit.

Through their remarkable ubiquity, J.K. Rowling’s series and the films based on it shaped an entire generation to an unprecedented and still unreckoned-with degree. Earlier touchstones—the Beatles or Star Wars, for example—may have marked epochal cultural moments, but Pottermania united kids across the globe, pulled in vast numbers of adult enthusiasts, and enjoyed the whole-hearted support of a well-coordinated and highly effective network of professionals—teachers, librarians, and booksellers among them. Certainly book publishing, usually regarded as a sleepy backwater of the culture industry, had never seen anything remotely like Harry Potter before (or since). Everything about the general frenzy, from the seemingly endless stream of media coverage to the hundreds of people lining up for midnight bookstore parties, came as a revelation.

Harry Potter also launched a phenomenon that’s seldom acknowledged and barely understood, but that’s as powerful and lasting as the books themselves: the first massive internet-born fandom. That fandom and its legacy have transformed today’s cultural landscape—creating new audiences, nurturing new billion-dollar media properties, and forever changing the way creators and consumers interact with each other and with the world.

Fandoms existed before Harry Potter, of course, going all the way back to the first truly modern fandom, which sprang up around Sherlock Holmes and harangued the great detective’s weary creator into resurrecting him from that supposedly fatal plunge off the Reichenbach Falls. Pottermaniacs were also far from the first fans to explore the internet’s ability to connect them with each other. Devotees of The X-Files are generally acknowledged to be pioneers, creators of the first internet-native community as well as inventors of the term shipping, used to describe fan activity (speculation, conversation, fan fiction) inspired by the desire to see two characters in the source material, or canon, become a couple.

But J.K. Rowling’s series came along at precisely the right historical moment to take advantage of the rapidly evolving internet. The first book, Harry Potter and the Philosopher’s Stone (to use the original British title), was published in 1997, followed by three more installments over the next three years. As Melissa Anelli, webmistress of the Leaky Cauldron, explains in her book Harry: A History, during roughly the same time span the number of Americans using the internet increased by more than fivefold, from 19 million to 100 million. Anelli writes, “The Internet changed Harry Potter about as much as the Internet was changing everything else.”

When she first got excited about Harry Potter in 2000, Tandy recollects, mailing lists and Usenet (an early form of discussion group) could be difficult for less internet-savvy people to access, especially because many fans tried to fly under the radar. “A lot of people thought being hard to find was important to fandom because if you didn’t, the Powers That Be would find out about you and shut you down,” Tandy said. Other groups shared fan fiction with adult themes and wanted to avoid exposing children to it. People who got very excited about, say, Star Trek were often met with ridicule and worse for their nerdiness, leading to shame or just an excess of caution when it came to talking about their taste.

Easy-to-use browsers like Netscape Navigator, first released in 1994, and the rapid adoption of the web suddenly made the internet’s riches available to anyone daunted by its original command-line interface. You could use the web to ask to join a Yahoo Group like the seminal Harry Potter for Grownups, but users on those lists—many of them still paying for their internet use by the hour—didn’t like to see their inboxes “cluttered up” with fan fiction and other “off-topic” material. “On forums,” Tandy explained, “you could talk about a lot more topics.” Forums were visible and more free-ranging than Listservs, divided up into categories and topics but also highly browsable. You might come to a web log like the Leaky Cauldron for news about forthcoming books and films and then wander off into its forums to join fans scrutinizing the books for clues or fiercely debating the motives of Severus Snape.

At times, the conversations could get pretty granular. “Ok, as weird as this may seem, this is meant to be a serious question!” wrote Holly in 2001 on MuggleNet, one of the first and biggest fan sites, founded by 12-year-old Emerson Spartz in 1999 and favored by younger readers. “What do the characters of Harry Potter wear under their robes? I figure they have to wear something, so do they wear muggle clothing or do they have a dress code?” Other burning topics included whether Dumbledore would ever tell Harry why Voldemort tried to kill him, whether Harry would play Quidditch professionally, and whether or not Ron “liked” Hermione based on his behavior at the Yule Ball.

Each volume in Rowling’s series is set during a single Hogwarts school year, and in the prolonged interlude between the 2000 publication of the fourth book, Harry Potter and the Goblet of Fire, and the fifth, Harry Potter and the Order of the Phoenix—a time that Harry Potter fans have named the Three-Year Summer—hunger for new stories about her beloved characters rose to a fever pitch. This led many curious young Pottermaniacs to discover fan fiction for the first time. In forums and especially in fan fiction communities, a fledgling writer could earn a modest or even substantive reputation and following of her own. It was during this lull in Rowling’s publication schedule that Tandy and fellow fans, discontented with the policies of the online clearinghouse fanfiction.net, founded FictionAlley. “It was a real ‘on the internet, nobody knows you’re a dog’–type situation,” Flourish Klink told me of the regular column she wrote on fanfiction.net starting in 1999. She was 12. The truth about her tender age only really came out two years later, when she joined Tandy in starting the nonprofit that runs FictionAlley: “We realized I couldn’t be on the board or sign any forms because I was too young.”

The web made the fandom a much bigger, better connected, and more international phenomenon—in high school, Klink counted as her best friend a fellow fan who lived in Switzerland—but Harry Potter lovers were already legion and unabashedly open about their enthusiasm. Because everyone they knew in real life was also reading the books, kids felt no hesitation to geek out over them, and in the process, geeking out in general lost much of its stigma, for good. Unlike comic books, rock ’n’ roll, and video games, Pottermania was a youth craze that inspired very little backlash from authorities, apart from the occasional religious nut who thought Rowling was promoting witchcraft. For younger children, this support meant getting to go to midnight release parties and having permission to socialize online with other fans. Adults who loved the books sometimes got a little more guff from their peers, but positive coverage in the Times and other establishment publications kept reminding them that they were far from alone. They started “Wizard Rock” bands and invented a real-life form of Quidditch that is now played by more than 500 teams in 26 countries.

Although Rowling did occasionally tangle with fans over sexually explicit fan fiction made too accessible to children or for-profit plundering of her own work, she went on the record as “flattered” by grassroots fan activities and fan-authored stories based on her books. Tandy—now a trademark attorney—was invited to the Warner Bros.’ studios in Burbank, California, to meet with their intellectual property counsel and found them “nice, friendly, and supportive of fans, fandom and fannish creativity.” Harry Potter fandom was the first large, multimedia fandom to flourish under such an imprimatur, and today most rights holders like film and TV studios actively encourage the sort of fan activity they once threatened to prosecute.

After the last volume in the Harry Potter series was published, in 2007, the book industry enjoyed a boom in young adult publishing; children’s books continue to be one of the sectors of business that most consistently thrives. Series like Twilight and The Hunger Games became hits that led to blockbuster films. Few realize just how big a role the fan networks and projects inspired by Rowling’s series have played in those successes. For example, the YA novelist John Green, author of The Fault in Our Stars, was already a prize-winning author when his brother, Hank, posted a YouTube video of himself warbling a song about the vexations of awaiting the publication of Harry Potter and the Deathly Hallows. But when that video—part of an ongoing series of public messages the siblings exchange as the vlogbrothers—went viral, it brought them to the attention of millions of Potter fans who in time also became Nerdfighters, as fans of the Greens call themselves.

It was the mass migration to LiveJournal by a significant portion of creatively inclined Potter fans in the mid-2000s that eventually led to the flourishing of networks that would transcend Harry Potter itself and foster the fledgling writers who would become the next generation of YA novelists. An early form of social media, LiveJournal combined a blogging platform with both a friending and a commenting system. You could use the site as it was originally intended, to keep and share a daily account of your thoughts and activities, but fan fiction writers also posted their stories there, and other members (depending on the account’s privacy settings) reviewed them; conversations and controversies blossomed. On LiveJournal, you could follow tags that kept you up to date on what other fans thought about the Harry Potter canon, but you could also follow particular writers, who in time developed fans of their own. The New York Times best-selling YA author Cassandra Clare started out writing Potter fan fiction before moving on to writing her own books.*

But above all, Potter fandom taught an entire generation of readers how to be enthusiastic evangelists for the books that capture their fancy, how to bypass traditional media and forge a new supercharged incarnation of word-of-mouth. While I was researching a 2012 piece about the publication of Suzanne Collins’ The Hunger Games, sources told me again and again that online networks of YA bloggers, vloggers, and social media mavens had played a major role in the book’s success, and that most of these “influencers,” all women in their 20s, had roots in Potter fandom. At the moment, it might be sites like ReadingTeen or Pandora’s Books or YA Interrobang that fans of the genre turn to, but none of these myriad voices have set themselves up as alternatives to establishment publications like the New York Times Book Review or Publishers Weekly; a more fundamental change has taken place. Most blogs, like most fan activity, are amateur projects. They come and go, flare into popularity for a year or two, then fall into inactivity when the blogger loses interest or goes off to college or has a baby. Even at its peak, no individual blog or vlog has the power to rocket a book onto the best-seller list all on its own. But this is the internet: Power comes in the aggregate.

Over the past two decades, fandom itself has gone from being about a passion for a particular story or set of characters to a kind of self-sustaining identity. LeakyCon, originally a Harry Potter convention organized by the Leaky Cauldron, has spawned GeekyCon, an annual gathering of people who love other books, movies, and TV series the way they love Harry Potter. Fannishness about Rowling’s books has morphed into fannishness in general, and even fannishness about fannishness itself. Instead of the preoccupation of a rather secretive, sheepish minority of people mocked for their geeky passions, fandom had become a badge of honor, even a claim to fame. It feels like home to vast numbers of people who grew up in the 2000s.

Harry Potter made possible this shift in attitude, and it also laid down connections and habits of connecting that have transformed how millions of people find out about their next favorite book, movie, or TV show. Fans graduated from grateful consumers of culture to a force to be reckoned with, capable of propelling a creator’s work into enormous popularity, but also prone to inflicting various forms of punishment when they are displeased. The fans of the BBC series Sherlock have been feuding with its producers, who alternately appease and bait them, for a couple of years now. Recently, fans enraged by an image posted by a storyboard artist for the animated series Steven Universe hounded the woman off Twitter. Quitting Twitter over harassment borne out of “fantitlement” has become something of a rite of passage for creators. Joss Whedon, of Buffy the Vampire Slayer and Avengers fame, ever the trendsetter, has done it twice.

Whether today’s fans really have much say in the direction taken by an unfolding series remains debatable. Most creators still fiercely protect their prerogatives when it comes to the work itself. But the new networked fandoms that emerged in the years since Rowling published that first Harry Potter novel have undeniable clout in the marketplace, courted and favored by studios and publishers alike. And nowadays, the very last emotion Heidi Tandy feels when she sees an email from a movie studio in her inbox is dread.

*Correction, Sept. 14, 2016: This essay originally noted that author Rainbow Rowell based her novel Carry On on Harry Potter fan fiction. Rowell wrote Slate to note that Carry On is not based on her previous work. (Return.)