According to Marx and Engels, relations between lords and serfs were characterized by oppression. The oppressor and the oppressed were bound together in a one-sided economic arrangement characterized by exploitation. The constant mutual antagonism between the two classes—sometimes hidden, sometimes out in the open—would eventually end in an open fight that would either bring about a revolutionary reconstitution of society as a whole or result in the common ruin of both classes.

In the case of medieval feudalism, the system's destruction led directly to the establishment of modern capitalism, with former serfs becoming either proletarians or members of the rising bourgeoisie. Under the new system, the very system that Marx and Engels wish to destroy, relations between these classes were still based on exploitation: the only thing that changed was the basis of that exploitation.

Unlike serfs, the proletariat are technically free in that they aren't tied to any one place. They can work wherever they like—in principle, at any rate. In actual fact, however, wherever they go and whatever they do to make a living, they will continue to be exploited by the bourgeoisie, who have now taken on the role in society previously occupied by feudal lords.