The theory of imperialism refers to a range of theoretical approaches to understanding the expansion of capitalism into new areas, the unequal development of different countries, and economic systems that may lead to the dominance of some countries over others. These theories are considered distinct from other uses of the word imperialism which refer to the general tendency for empires throughout history to seek power and territorial expansion. The theory of imperialism is often associated with Marxist economics, but many theories were developed by non-Marxists. Most theories of imperialism, with the notable exception of ultra-imperialism, hold that imperialist exploitation leads to warfare, colonization, and international inequality.

Imperialism is the practice, theory or attitude of maintaining or extending power over foreign nations, particularly through expansionism, employing both hard power and soft power. Imperialism focuses on establishing or maintaining hegemony and a more or less formal empire. While related to the concepts of colonialism, imperialism is a distinct concept that can apply to other forms of expansion and many forms of government.