First LEGO System themes (1978–1999)
With the introduction of the minifigure-compatible System line in 1978, LEGO established the three major themes Town, Castle and Space, inside each of which were later added several new subthemes, that mostly consisted of new distinct factions of minifigures and were all related to the respective main theme's core subjects.
With Boats and Trains, two minor themes where introduced that complemented Town but were not counted as parts or subthemes of it. Another unique theme was Model Team from 1986, whose sets focussed on more or less accurate scale reproductions of real world vehicles and were not designed to be minifigure-compatible.
Beginning with Pirates in 1989, LEGO added additional main themes to the LEGO System line that stood beside the three initial main themes. These four major theme made up the bulk of the LEGO System sets of that era and were also the most popular themes. Another minor System theme Belville was introduced in 1994, which contained larger figure and more specially-designed bricks. The next addition was Aquazone in 1995 followed by Western in 1996. The odd Time Cruisers line from the same year is considered a subtheme of Town.
The, now six, major minifigure-themes (Town, Castle, Space, Pirates, Aquazone and Western) were subjected to bigger changes in 1999: Space, Pirates, Aquazone and Western were discontinued for the time being. Castle was rebranded to Knights' Kingdom. Town's core subjects were outsourced to Town Jr. and later City Center, while still being complemented by different subthemes which were in most cases only remotely related to city-life.
Smaller themes and licensing (1999 onwards)
Instead of slowly adding new factions and sets to existing main themes, most themes were now self-contained series of sets that in most cases had all of its sets released in its first year and only lasted for one or two more years. One of these new themes was Rock Raiders from 1999 and Life on Mars from 2001, which was a shortlived revival of the old Space theme.
- Small themes
- Rock Raiders (1999)
- Life on Mars (2001)
- Island Xtreme Stunts (2002–2003)
- World City (2003–2004)
- Dino Attack (2005)
- Vikings (2005–2006)
- Aqua Raiders (2007)
The other side of the medal were themes that were based on popular media franchises like Star Wars and Harry Potter. Those themes would regularly see new releases over the timespan of several years. Another possibility of releasing sets based on licenses was the Studios theme.
- Star Wars (1999 onwards)
- Harry Potter (Theme) (2001–2007), (2010–2012)
- Batman (Theme) (2006–2008)
- SpongeBob SquarePants (Theme) (2007–2009), (2011)
- Indiana Jones (Theme) (2008–2009)
- Studios: Jurassic Park III, Spiderman
- Toy Story / Toy Story 3 (2010)
- Cars (2010)
- Pirates of the Caribbean (2011)
- Super Heroes: DC, Marvel (2012)
Nevertheless, there were still some original LEGO themes that ran for several years and were regularly updated with new sets, like Alpha Team that ran from 2001 to 2005 and to a smaller extent Knights' Kingdom II that saw new releases from 2004 to 2006.
Relaunch of classic themes (2005 onwards)
With the introduction of City in 2005, the constant rebranding of the Town theme in the years after 1999 came to a halt. It also marked the first true revival of one of the classic major themes since the initial concept of complementing a consistent long running theme by regular releases of new sets saw a renaissance as well.
Other recent themes were Castle (2007) and the continuation of Pirates. Space saw a partial revival in the form of Mars Mission (2007 - 2008) and then again with Space Police from 2009 - 2010, and then later in Alien Conquest (2011) and Galaxy Squad in 2013.