A popular front is "any coalition of working-class and middle-class parties", including liberal and social democratic ones, "united for the defense of democratic forms" against "a presumed Fascist assault". More generally, it is "a coalition especially of leftist political parties against a common opponent". The phrase uses "front" in the sense of a political movement "linking divergent elements to achieve common objectives".

The term was first used in the mid-1930s in Europe by communists concerned over the rapid growth of fascist movements in Italy and Germany, which they sought to combat by coalescing with non-communist political groupings they had previously attacked as enemies. Temporarily successful popular front governments were formed in France, Spain, and Chile in 1936.

The name has also been used by other alliances such as the Popular Front of India. In the late years of the Soviet Union, the popular fronts created actually played a key role in ending Communist Party rule in the Soviet republics.

When communist parties came to power after World War II in the People's Republic of China, and the countries of Central, and Eastern Europe, it was common to do so at the head of a "front" (such as the United Front and Chinese People's Political Consultative Conference in China, the National Front in Czechoslovakia, the Front of National Unity in Poland, the Democratic Bloc in East Germany, etc.) containing several ostensibly-noncommunist parties. While it was the communist party—not the fronts—that held power in these countries, the alleged coalitions allowed the Party to deny that it had a monopoly on power.

A related term was "Communist front", meaning an organizational facade used to mask the true character of "the actual controlling agent", the Soviet Communist Party, with no real influence by others. The strategy of creating or taking over organizations that would then claim to be expressions of popular will, not merely organs of the Communist Party, was first proposed by Vladimir Lenin. Rather than political coalitions opposing fascism, these groups sought to spread the Marxist–Leninist message in places where it was either illegal or distrusted by many of the people the party wanted to reach.

Front organizations were used from the 1920s through the 1950s, and proliferated during the popular front political coalitions of the 1930s. Eventually there were large numbers of front organizations, such as the World Federation of Democratic Youth, International Union of Students, World Federation of Trade Unions, Women's International Democratic Federation, and the World Peace Council. Anti-communists during the Cold War frequently accused liberal political organizations of being Communist fronts.

The Communist International (Comintern), the international organization created by the Soviet Communist Party in the wake of the 1917 Bolshevik Revolution, went through a number of ideological strategies to advance proletarian revolution. Its 1922 congress called for a "United Front" (the "Second Period") after it became clear that proletarian revolution would not imminently sweep aside capitalism in the rest of the world: the minority of communist revolutionary workers would join with workers outside the communist parties against the bourgeoisie. This was followed by the "Third Period" starting in mid-1928, which called for militant policies to take advantage of the economic crises of capitalism, with no need for coalitions with non-communists. As the Nazi Party came to power in 1933 in Germany and eliminated the powerful German communist movement, it became clear that fascism was the main enemy, and that opposition to it was disorganized and divided. A new, less extreme policy was needed, whereby Communists would form political coalitions with non-Communist socialists and even democratic non-socialists – "liberals, moderates, and even conservatives" – in "popular fronts" against fascism.

Until early 1933, the Communist Party of Germany (KPD) was regarded as the world's most successful communist party in terms of membership and electoral results. As a result, the Communist International, or Comintern, expected national communist parties to base their political style on the German example. That approach, known as the "class against class" strategy, or the "Third Period", expected that the economic crisis and the trauma of war would increasingly radicalise public opinion and that if the communists remained aloof from mainstream democratic politics, they would benefit from the populist disillusionment and be swept to power. Non-communist socialist parties were denounced as "social fascist".