Since the birth of the labour movement to this day, an intense struggle between two tendencies has been waged within the movement: the revolutionary one and the opportunist one. Over the history, opportunism has adopted different and numerous expressions, diguised under forms of "left wing" and right wing. This article deals with the right wing opportunism or revisionism, initial source of the political current that is nowadays known as social-democracy, whose nature mutated along the twentieth century, from being a current of the labour movement to a political movement which is an uncompromising defender and the essential pillar of monopoly capitalism.
Revisionism emerged in the late nineteenth century when, after the passing away of Frederick Engels, open warfare broke out within the socialist movement led by the German Eduard Bernstein whose maxim “the movement is everything, the ultimate aim is nothing”  became the banner of the followers of the revisionist theory and its political practice, reformism. Lenin would argue about it:
“This catch-phrase of Bernstein’s expresses the substance of revisionism better than many long disquisitions. To determine its conduct from case to case, to adapt itself to the events of the day and to the chopping and changing of petty politics, to forget the primary interests of the proletariat and the basic features of the whole capitalist system, of all capitalist evolution, to sacrifice these primary interests for the real or assumed advantages of the moment—such is the policy of revisionism. And it patently follows from the very nature of this policy that it may assume an infinite variety of forms, and that every more or less “new” question, every more or less unexpected and unforeseen turn of events, even though it change the basic line of development only to an insignificant degree and only for the briefest period, will always inevitably give rise to one variety of revisionism or another.” 
Revisionism, claiming that the socio-economic conditions had changed radically, expressed itself as a current openly hostile to Marxism, rejecting the basic postulates of Marxist science:
- In the sphere of philosophy, it denied its partisan and class character, being in tow of the bourgeois “science” and dragging b along after the “neo-Kantian” thinkers. 
- In economic terms, it denied the theory of value, the law of capitalist accumulation and the law of absolute and relative impoverishment of the proletariat in the new conditions of capitalism. It was said that concentration and the ousting of small-scale production by large-scale production does not occur in agriculture at all. They defended the idea that the process of concentration of ownership proceeded very slowyly in commerce and industry. They expressed the view that the big capitalist companies would end the anarchy of production and therefore reduce automatically the contradiction between the proletariat and the bourgeoisie. 
- In the sphere of politics, revisionism sought to review what actually constitutes the basis of Marxism: the theory of class struggle. Political freedom, democracy and universal suffrage remove the ground for the class struggle —we were told by the revisionists—. For, they said, since the “will of the majority” prevails in a democracy, one must neither regard the state as an organ of class rule, nor reject alliances with the progressive, social-reform bourgeoisie against the reactionaries. 
For Lenin, revisionism —revision of Marxism— was one of the chief manifestations, if not the chief, of bourgeois influence on the proletariat and bourgeois corruption of the workers.  In his workThe Collapse of the Second International, he gave the following definition of opportunism:
“Opportunism means sacrificing the fundamental interests of the masses to the temporary interests of an insignificant minority of the workers or, in other words, an alliance between a section of the workers and the bourgeoisie, directed against the mass of the proletariat.” 
The fact is that ideology is the reflection, in the consciousness of human beings, of the objectively existing social conditions, and mainly a reflection of the prevailing production relations. Thus, from the Leninist view, the historical roots of the revisionist phenomenon and its class nature are highlighted:
“In every capitalist country, side by side with the proletariat, there are always broad strata of the petty bourgeoisie, small proprietors. Capitalism arose and is constantly arising out of small production. A number of new “middle strata” are inevitably brought into existence again and again by capitalism (...) These new small producers are just as inevitably being cast again into the ranks of the proletariat. It is quite natural that the petty-bourgeois world-outlook should again and again crop up in the ranks of the broad workers’ parties. It is quite natural that this should be so and always will be so, right up to the changes of fortune that will take place in the proletarian revolution.” 
In short, Marxism-Leninism emphasizes three essential particularities of right-wing opportunism or revisionism:
- Revisionism is an international phenomenon, being a social product of a particular historical epoch.
- Revisionism regularly appears in the workers' parties, given the cyclical nature of capitalist development, and it can adopt diverse forms.
- Right-wing opportunism, in reviewing the basic postulates of Marxism, distorts the revolutionary character of the workers' party, deviating it from its main objective: the destruction of the economic and political power of the bourgeoisie. 
Faced with the reformist political practice that stems from the revisionists theoretical standpoints, Lenin argued that the bourgeoisie grant reforms with one hand, and with the other always take them back, reduce them to nought, use them to enslave the workers, to divide them into separate groups and perpetuate wage-slavery. For that reason reformism, even when quite sincere, in practice becomes a weapon by means of which the bourgeoisie corrupt and weaken the workers. The experience of all countries shows that the workers who put their trust in the reformists are always fooled.