When it comes to contemporary political theory， people constantly make comparison between works of Foucault and Marx. While most theses focus on Foucault’s and Marx’s theory of power， this essay discusses their theory of population—an important concept in both people’s works. This essay first introduces the word’s origin “populousness”. In the body part， it analyzes the background of the two people’s theories， how they understand the concept population and how they think population works. From the comparative analysis it can be concluded that the concept population has changed in many aspects from an economic context into a political context over a hundred year.
　　Populousness
　　The word population origins from Latin word “populus” and populousness has a long and complicated history in western world， which showed up as early as in Aristotle’s Politics. “It points to the sense that units of government （kingdoms， empires， countries， parishes， cities） contain greater or lesser number of entities—hearths， soldiers or souls， for instance—distributed across different orders or classes” （Curtis 508）. The concept populousness makes it possible to plan the future and the degree of it reflects the index of wealth and measure of policy. Basically populousness involves numbering people， and more importantly， it “implies hierarchical differentiation of orders of the people.”（Curtis 508）
　　Population of Marx
　　In the early and mid 19th century， Europe witnessed a massive storm of first industry revolution where great changes of every social aspect took place. Engels pointed out that with the advent of the machine， unemployment as well as “want， wretchedness， and crime” all showed up. What is more， he noticed there was a gigantic expansion of population at that time， which can be regarded as first formulation of Karl Marx’s theory of population. Marx theory of population is also called theory of surplus population since he introduces “relative surplus population” in Capital：
　　The laboring population therefore produces，， along with accumulation of capital produced by it， the means by which [it]itself is made relatively superfluous， it turned into a relative surplus population； and it does this to an always increasing extent.（Marx 692）
　　According to Marx， the intention of the phrase “surplus population” is to allege that it is not nature but productive system and material condition that cause people surplus. It can be explained in The German Ideology：