Since the late 1930s, SF fans have organized conventions, non-profit gatherings where the fans (some of whom are also professionals in the field) meet to discuss SF and generally enjoy themselves. (A few fannish couples have held their weddings at conventions.) The 1st World Science Fiction Convention or Worldcon was held in conjunction with the 1939 New York World's Fair, and has been held annually since the end of World War II. Worldcon has been the premier convention in fandom for over half a century; it is at this convention that the Hugo Awards are bestowed, and attendance can approach 8,000 or more.
SF writer Cory Doctorow calls science fiction "perhaps the most social of all literary genres", and states, "Science fiction is driven by organized fandom, volunteers who put on hundreds of literary conventions in every corner of the globe, every weekend of the year."
SF conventions can vary from minimalist "relaxacons" with a hundred or so attendees to heavily programmed events with four to six or more simultaneous tracks of programming, such as WisCon and Worldcons.
Commercial shows dealing with SF-related fields are sometimes billed as 'science fiction conventions,' but are operated as for-profit ventures, with an orientation towards passive spectators, rather than involved fans, and a tendency to neglect or ignore written SF in favor of television, film, comics, video games, etc. One of the largest of these is the annual Dragon*Con in Atlanta, Georgia with an attendance of more than 20,000 since 2000.