Since its making scientific socialism distinguished itself from other theories identifying within the existing society a social force called to bury capitalism and to erect a new society. This social force was the working class. Since the first Marxists works, since the first manuscripts, for example The situation of the Working Class in England, The Communist Manifest or Principles of Communism “the main thing in Marx and Engels’ doctrine was the clarification of the historic role of the proletariat as creator of a socialist society”. 
Marx and Engels based these assertions in a deep analysis of capitalist economy. What are the conditions, characteristics and qualities fulfilled by the working class to be called to carry out this role?
In first place it is the most exploited class in capitalist society. Their life conditions are determined by the fact that their existence, joys and sorrows, life and death depend exclusively on the sale of their labor force to capitalist and of the conditions of this sale given by market fluctuations. This life conditions, this vital interest propels them to constantly fight to death with the capitalist class, this turns the proletariat into the most consequent and firm adversary of the capitalist system.
This is not purely an empirical observation; it is based in the discovery of the surplus value theory, which conserves full validity. The present outbreak of the capitalist economic crisis of overproduction and over accumulation has come to destroy the last delusions of those who thought that in economy the sphere of circulation could develop independently of the sphere of production and the laws that govern it.
In second place in that the working class is linked to the development of the productive forces. As workers, they have no links to the past of production, with the remnants of past productive systems, but with the development and the future of production.
This means, in countercurrent of many assessments, that the material development of capitalism, great industry, does not threaten the existence of the proletariat as a class, it does not destroy their positions in society; it impels the numeric growth of the workers and widens their role in social life.
It is methodologically groundless to take a very short period of time to make assessments over the disappearance of the proletariat. The law of proletarization of the population shows its impressive reach when we analyze capitalism as a whole. For example, in the mid XIX century in the United States the working class, workers and their families, constituted less than 6% of the population, in Germany it raised no more than 3%, at mid XX century this cipher had grown up to half the population in both cases. Nowadays, according to ILO figures, at a global scale, the class of the workers disposed of means of production and that sell their labor force in exchange for a salary has oscillated since the 80’s around 65% of the population.
This means that the interests and aspirations of the working class coincide with the general orientation of the development of the productive forces. The level of development achieved by the productive forces requires the suppression of the private property over the means of production. In fact this is announced by the relative suppression of private property over the concentrated and centralized means under the very framework of capitalism since the rise of anonymous societies and monopolies.  Lacking all private property over the means of production the working class can’t have great esteem for it. Rather, private property over the means of production is the basis for the exploitation of the worker by the capitalist, which is why its suppression and substitution by social property is the only way the working class has for emancipating itself.
The fact that besides this the working class counted with qualities, derived from its position in production, which were regarded as indispensable for a revolutionary labor did not escape to the masters of scientific socialism.
For example, we have talked already about its constant growing number, the proletarian movement – said Marx & Engels in the Communist Manifesto- is the independent movement of the huge majority in interest of the huge majority.
But it is not only about an aspect of quantity, also the own bourgeoisie when concentrating means of production gathers thousands of workers under the roof of its factories, ordinarily located in poles of capital concentration, that is, the large cities. Thus the proletariat overcomes dispersion and Isolation. As problems of the subjective order are surpassed and the level of class consciousness rises, the workers can unite and organize better than any other class.
This concentration of the working class is independent of certain temporal developments. There can be periods of time and countries in which a section of the capitalist opts for decentralization or sectioning of the productive process. This obeys generally to conditions under which it is convenient to capture new surplus value through this way, or also, to disperse temporally the working class and difficult its organization when the sacrifice is deemed necessary. However this option is reverted after a time, the general process shows that the tendency of capital is towards concentration. This is witnessed by the uninterrupted growth of monopolies, the fact that an ever greater percentage of the working class works directly for them and its reflection on the uninterrupted growth of urban concentrations.
Moreover the working class is the class that is most ready, out of its own conditions, for organization. The work in the great enterprises accustoms the worker to the spirit of collectivism, to a sever discipline, to joint actions and to solidarity. For example Engels talks about this stringent discipline, with the adjective of military, in The situation of the Working Class in England, Lenin underlines in his Notebooks on Imperialism how the capitalist accustom the working class to an extraordinary precision in each movement. And all this before the vigilance and control that the new information and telecommunication technologies allow!
Of all the oppressed classes, the working class is the most capable of developing its consciousness and accepting a scientific ideology. The advance of industry has required of ever more educated workers. The handling of the valuable and complex machines in which production is sustained today requires a high degree of scientific preparation and a cultural level much higher than preceding stadiums of economic formations.
It is the sum of all this historic and economic conditions which turn the working class in the most militant and revolutionary class of society. These historic and economic conditions keep full force in our days.