(written from a Production point of view)
Star Trek conventions are organized gatherings of Star Trek fans. The larger ones usually have stars and production staff speak or at least sign autographs. It is also common for there to be booths where vendors will sell Trek-related merchandise or collectors will sell and trade. Most of the formerly Trek-only conventions of the mid '90s have now expanded to include other sci-fi and fantasy series.
The first Star Trek convention
The convention held in New York from 21-23 January 1972 is often recognized as the first true Star Trek convention, even though Star Trek: The Original Series had already made its presence felt at earlier (as early as its second season, then still being in production, and ultimately leading up to the establishment of Lincoln Enterprises), more generic science fiction conventions. The earliest known such presence was the 1–5 September 1966 "Tricon World Science Fiction Convention" in Cleveland, Ohio, where Gene Roddenberry promoted the new Star Trek series, slated to start airing the subsequent week, as well as presenting the audiences the first two pilot episodes for the series, "The Cage" (uncut original version) and "Where No Man Has Gone Before" (revised version, as this episode was already slated to air). (The Star Trek Compendium, 1993, pp. 1-3) To this date, the franchise continues to make its presence felt outside the specialized Star Trek convention circuit as well, such as the hugely popular ComicCons.
The specialized "true" Star Trek conventions were started by a small group of Trekkies, commonly referred to as "The Committee" – not to be confused with the similarly named one, organized by Roddenberry and several science fiction writers in 1967, for the very first "save Star Trek" letter campaign (not the famed 1968 one) – , who combined their money, and rented a hotel ballroom, in the hope of getting a group of like-minded fans together. Committee member Joan Winston, whose main responsibility was that of the dealer's room, gives a detailed account of the first convention in Star Trek Lives!. Other members included Allan Asherman, Eileen Becker, Elyse Pines, Steve Rosenstein and Al Schuster.
Although the original estimate of attendees was only a few hundred, several thousand had turned up before the end of the convention, which featured a program of events of an art show, costume contest, a display provided by NASA and a dealers room. Episodes were also screened from 16mm prints, including the original pilot "The Cage" and blooper reel. A number of Trek-connected guest speakers also attended including Trek's creator, Gene Roddenberry, Majel Barrett, D.C. Fontana, as well as science fiction author Isaac Asimov. One of the most surprising guest speakers, had been former Desilu executive Oscar Katz in his one and only public Star Trek appearance, recounting the days when Roddenberry made his Star Trek is... pitch to him and his colleague Herb Solow and the subsequent efforts to sell the series to the networks. (Marketing the Moon: The Selling of the Apollo Lunar Program, 2014, p. 3)
After this gathering, a series of annual events was organized, which soon included regular cast members, of which DeForest Kelley, Nichelle Nichols, Leonard Nimoy, George Takei, and James Doohan give their own account in the film Trekkies.
Star Trek 30: One Weekend On Earth
Star Trek 30: One Weekend On Earth was held 7 September 1996 – 8 September 1996 in Huntsville, Alabama at the US Space and Rocket Center and NASA's Marshall Space Flight Center. The event was attended by each of the casts and crews, including the entire cast of The Original Series – the last time they all appeared on stage together. This convention was also the last such event Star Trek's licensing arm, then Viacom Consumer Products, directly produced. 
FedCon, short for Federation Convention, is Europe's largest annual sci-fi convention, held in Bonn, Germany (occasionally it was held in Düsseldorf and Fulda). It started in 1992 as a pure Star Trek convention, but has expanded to include various other film and television series. The film Trekkies 2 visited FedCon XI (2003), and FedCon XV (2006) was also a stop on the international tour of the 40 Years of Star Trek: The Collection exhibition. Attendance at FedCon XI averaged 5,000 per day.
Creation Entertainment was founded in 1971, and has been producing "traveling" Star Trek conventions for many years in cities all over North America and Great Britain. At the height of Star Trek's popularity in the early to mid '90s, Creation was organizing 110 conventions per year, sometimes three in one weekend. They have a long licensing history with Paramount and Viacom Consumer Products, and have sold nearly US$12 million of official Trek merchandise. Creation started its association with Star Trek in 1991, when it attained a license from Paramount "for the production of film stills", encompassing a landmark agreement for the entire franchise that covered photos as well as a wide range of collectible souvenirs. 
The company became hosts of the annual Official Star Trek convention – first with the "Grand Slam" event each spring in Los Angeles and then Pasadena, California, which ran from 1993 to 2008, with a revival in 2013. A segment of Journey's End: The Saga of Star Trek: The Next Generation was filmed at the 1994 Grand Slam.
Beginning in 2002, Creation started a second annual convention at the Las Vegas Hilton (in conjunction with Star Trek: The Experience) in August. By 2005, 15,000 people bought tickets to the convention. The event, often abbreviated to "VegasCon" or the hashtag #STLV, moved to the Rio All-Suites Hotel and Casino in 2011, two years after The Experience closed.
The 2006 40th anniversary convention was held in Las Vegas on 17 August through 20 August. It was a stop on the international tour of the 40 Years of Star Trek: The Collection exhibition, and featured in the documentary Star Trek: Beyond the Final Frontier produced by The History Channel. For the 2011 45th anniversary, the 11 through 14 August Las Vegas convention  was preceded by two smaller ones in San Francisco (11-13 March ) and Boston (15-17 ). Five years later, the 50th anniversary convention was a complete sellout for the first time ever, and had expanded to five days, from 31 July through 4 August. 
The 2013 Vegas convention saw a world record attempt for the largest gathering of people in Star Trek costume. 1085 attendees, including guest Terry Farrell, beat the record established at Destination Star Trek London the previous year. 
Destination Star Trek
Running since 2012 on a biennial basis (with two events in 2014), Destination Star Trek is the official European Star Trek convention. The convention has taken place in London (twice), Birmingham (twice), and Frankfurt.
Vulkon began running conventions in 1987 in places like Orlando, Cleveland, Houston, and Atlanta. Vulkon featured a banquet with the stars, a live DJ and dance on Saturday evenings, and a costume contest. Normaly two Vulkons were held each year in Orlando, Florida – one in the spring and one in the fall – with an average attendance of between 1,500 and 2,000 people. In late 2008, the company filed for bankruptcy and subsequently dissolved.
Chris Avilla was a Vulkon convention staffer before he became a prop maker for several Star Trek productions.
- "The Conventions as Asimov Sees Them", Isaac Asimov, Starlog, issue 1, August 1976, p. 43
- "The Star Trek Bi-centennial-10 Convention" (Special section), Starlog, issue 3, January 1977, Joan Winston, Jim Burns, et al., pp. 24-39
- The Making of the Trek Conventions, 1977
- "A Trek Cruise Down Memory Lane", Sue Uram, Cinefantastique, Vol 22 #3, 1991, pp. 19-22