An empire involves the extension of a state's sovereignty over external territories. For example, first the Spanish Empire and then the British Empire were called "the empires on which the sun never sets", because of their territories and possessions around the globe. This article provides a list of the largest empires in world history.

There are various notions of size that can be applied; for each of these notions it is the case that for most historical empires only estimates can be given for the maximal value in time attained for that particular notion of size, and usually there is furthermore no clear consensus among historians regarding the best estimate – if only because there is often no unambiguous information about an empire's historical boundaries. Thus, the values given here should generally be interpreted as being only indicative, and not as determining a precise ranking.

The calculation of the land area of a particular empire is controversial. In general, the list centers on the side of including any land area that was explored and explicitly claimed, even if the areas were populated very sparsely or not at all. For example, a large portion of Northern Siberia is included in the size of the Russian Empire but not the Mongol Empire. The Mongol Empire's northern border was somewhat ill-defined, but in most places it was simply the natural border between the steppe and the taiga. At the time the majority of the taiga and tundra were unexplored and uninhabited. This area was only very sparsely populated by the Russian Empire, but it had been explicitly claimed by the Russian Empire by the 17th century, and its extent had been entirely explored by the late 19th century. Similarly, the northernmost Canadian islands such as Ellesmere Island were explored and claimed by the British Empire by the mid-19th century (virtually the entire mainland was at least sparsely populated well before that).