Before 1978, Lego produced several construction sets with common themes, but they were not necessarily branded as part of a single series or theme.
Following the introduction of minifigures in 1978, owner Kjeld Kirk Kristiansen pushed a new strategy of creating and marketing a series of sets he termed a "system within the system"[1] and the three original environments (based on the present, past and future, respectively) were launched: City/Town, Castle,[2] and Space.
[3] In 1987, Lego created sub-themes within these environments, as well as introducing branding that identified a set as part of a theme.
The company also produced product lines that used pieces outside of the standard Lego system such as Technic, Duplo and Fabuland.
Since then, many new themes have been introduced and discontinued, including the inclusion of licensed themes in 1999 such as Star Wars, Wizarding World or Marvel and DC Comics.