The mass line is a political, organizational, and leadership methodology developed by Mao Zedong and the Chinese Communist Party (CCP) during the Chinese Communist Revolution. Who used the term first is disputed, with some crediting Li Lisan and others Zhou Enlai. In mass line methodology, leadership formulates policy based on theory, implements it based on the people's real world conditions, revises the theory and policy based on actual practice, and uses that revised theory as the guide to future practice. This process is summarized as leadership "from the masses, to the masses", repeated indefinitely.

Mao developed the mass line into an organizing methodology that encompasses philosophy, strategy, tactics, leadership, and organizational theory, which has been applied by many communists subsequent to the Chinese Communist Revolution: from Che Guevara in Latin America, to Ho Chi Minh in Vietnam. Many CCP leaders have attributed their attainment of power to the faithful pursuit of effective "mass line" tactics, and a "correct" mass line is supposed to be the essential prerequisite for the full consolidation of power.

In its original conception, the mass line referred to both an ideological goal as well as a working method based on "pooling the wisdom of the masses" (simplified Chinese: 集中群众智慧; traditional Chinese: 集中群眾智慧; pinyin: jízhōng qúnzhòng zhìhuì) from which CCP leadership could formulate policy after further deliberation, adjustments, implementation and experimentation, which would in turn continue to receive feedback from the masses. The methodology is as follows:

Thus, the mass line is a method in which theory is refined by practice, with leadership flowing "to the masses - from the masses - to the masses".

In doing this, a line of feedback is formed between leaders and masses, representing the aggregate interests of ordinary people on their behalf, in a Maoist political line, ostensibly derived "from the peasants". It is government's role to listen to the scattered ideas of the masses, turn them into systemic ones, and return them back to the people as a guide for action. This is a process by which leadership refines the views of the people, "pooling the wisdom of the masses", while constantly adjusting and testing decisions in an "endless spiral" of improvement.

Pragmatic considerations evident in mass line include its ability to reconcile central leadership with mass consultation. It alleviates "two problematic tendencies" that occur with centralization: losing touch with popular sentiment and creating political apathy among citizens. Some have gone as far as to speculate that Mao's success resulted from his understanding of how government can employ mass line to its strength.

Mass line is also part of a long spanning Marxist–Leninist "epistemology (zhishi lun) or methodology (fangfa lun)". Mao acknowledged inspiration in the October Revolution and subsequent formation of Vladimir Lenin's Vanguard Party. Mass line also exhibits elements of ancient Chinese beliefs, which emphasized the importance of wise rulers reading signs of popular discontent in order to avoid social calamity. Some argue that Mao's conception of the mass line reflected his faith in the people as well as a theory of "history from below."

The principle of the mass line is reflected in the Party slogan "serve the people".