This chapter presents a critical review of the major Marxian theories of imperialism by tracing out the debates and examines the development of their basic tenets. Imperialism as a category in Marxian literature is theorised as a tendency specific to the post-competitive phase of capitalism. The chapter traces the continuities and departures that evolved in the realm of theory. It deals with Rosa Luxemberg's discussion on accumulation and the theory of imperialism that follows, because it is unique in conceiving imperialism as intrinsic to capitalism, which had been the predominant notion within Marxian discourse during pre- and post-war phase of inter-capitalist rivalry. Luxemburg's theory of imperialism as a structural inevitability was primarily driven by a political need of countering Karl Kautsky's revisionist position. Bukharin Lenin's imperialism is also a radical departure from the sterile evolutionary perspectives of the Second International. The notion of imperialism has to be problematised in the context of new realities.