The ideas of Marx and Engels on population are seldom dealt with, at least in the versions available in English or French. Their demostrations are backed up by specific examples taken from XIXth century England, since Marx constantly tried to find proofs for his theories. Engels had done the same to demostrate the universality of his principies on population. Marx's mose relevant studies were based on the work of economists and demographers; a third approach, the philosophical one was then proposed by the Althusserian school. This paper aims at making a synthesis of the contributions of Althusser's viewpoints on population on Marx himself: if the epistemiological breach leaves no doubts, it is then quite more complicated to become aware of the concept of population in his book, Lire Capilal (Reading Capilal).