Reformism is the belief that gradual changes through and within existing institutions of a society can ultimately change a society's fundamental economic relations, economic system, and political structures. This belief grew out of opposition to revolutionary socialism, which contends that revolutions are necessary for fundamental structural changes to occur.

Socialist reformism, or evolutionary socialism, was first put forward by Eduard Bernstein, a leading social democrat. Reformism was quickly targeted by revolutionary socialists, with Rosa Luxemburg condemning Bernstein's Evolutionary Socialism in her 1900 essay Reform or Revolution?. While Luxemburg died in the German Revolution, the reformists soon found themselves contending with the Bolsheviks and their satellite communist parties for the support of the proletariat.

After the Bolsheviks won the Russian Civil War and consolidated power in the Soviet Union, they launched a targeted campaign against the Reformist movement by denouncing them as "social fascists". According to The God that Failed by Arthur Koestler, a former member of the Communist Party of Germany, the largest communist party in Western Europe in the Interwar period, communists, aligned with the Soviet Union, continued to consider the "social fascist" Social Democratic Party of Germany to be the real enemy in Germany, even after the Nazi Party had gotten into power.