A convention (abbreviated: Con or Expo), is a term for a congress or assembly.
Previously a niche event especially for comics and fans of comic figures the great Hollywood studios have discovered Conventions as a great promotional tool for new tv shows and movies. The San Diego Comic Con held in June and the Wondercon organized by the same organizer, are now the largest conventions in the world next to the Dragon*Con in Atlanta. Especially the San Diego Comic Con (abbreviated: SDCC) has hardly to do anything at all anymore with comics only since in the last years the number of panels held for genre films and series increased tremendously, in order to promote new seasons or movie releases.
With the growing popularity of the X-Files, the X-Philes' demand for merchandise and events around the series, their makers and stars - not only in North America, but also in Europe and Australia - also rose.
Official / Licensed Events
The first official conventions with guest stars were held in Seattle in 1995 and 1996 in Burbank and Seattle. The convention in Burbank represented the first event with a leading actor present, Gillian Anderson.
1998 followed a large convention roadshow with the name The X-Files EXPO, which led through 10 large US-American cities. Those were organized by Creation Entertainment, Inc, and were licensed by 20th Century Fox. The mostly two-day events cost $ 25 per day to attend. For this one could, among other things, take his picture in Mulder's cellar office, look at props and clothes from the series, watch a gag reel, research things in the interactive The X-Files - Unrestricted Access Lexicon, compete against other fans in a trivia quiz and much more. Highlights were the questionnaires and panels with some of the actors and producers. Mostly military venues were used.
After that, the official events were held for 10 years. Some fan clubs and fan initiatives tried to set up independent conventions, in the case of the German and English fans with more and in case of the Australian fans with less success.
During the promotion for I Want To Believe, the second X-Files feature film, this time not only did Chris Carter and Frank Spotnitz attend various conventions, but also Gillian Anderson, and for the first time, David Duchovny, who had always opposed such events .
On the occasion of the twentieth anniversary of the premiere of The X-Files', Gillian Anderson went on an international convention tour in 2012 and 2013 to England, Canada and the USA. At the end of each panel, she auctioned a T-shirt (either with the COTR or ATTHS print) worn by her, in favor of one of the many charitable organizations she supports .
Since the official conventions were held mostly only in large North American cities, some fans very early were very active and started to organize conventions in their countries. The Germany based European X-Files Fan-Club was extremely busy and organized a convention in 1996, which was then also advertised in the tele text of the German Channel ProSieben. After the first event in Frankfurt, a second event took place in Cologne in 1997. 
The English fans were also busy: they held a convention in Manchester in 1997 and one in Coventry.
The official Australian fan club The X Files Fan Club of Australia, planned a convention, which never took place, for the main actress Gillian Anderson 1996 visited the country in the course of a promo tour for the Australian TV channel FOXTEL.
In the year 2000 a convention / fan meeting was planned in the Munich area of Germany, but never took place.
In 2009 the X-CON was created by a fan with the help of the German Internet community of xfiles-mania.de. After more than a year of organization work, the first X-CON with guest star Dean Haglund and Jamison Young took place in October. A fan meeting was organized in Hamburg in 2010 by the same organizer. The special feature of this event is that all proceeds were donated after deduction of the costs.
In Xanten, the X-Files Academies  took place regularly between 1997 and 2001. Fans had rented a house several times for a weekend where they did FBI training, shooting exercises, fitness, hostage negotiating, telephone surveillance and the like. At night, X-Files video games were played and movies and episodes were watched.