Legalism or Fajia is one of the six classical schools of thought in Chinese philosophy. Literally meaning "house of (administrative) methods / standards", the Fa "school" represents several branches of "men of methods", in the west often termed "realist" statesmen, who played foundational roles in the construction of the bureaucratic Chinese empire.

The earliest persona of the Fajia may be considered Guan Zhong (720–645 BCE), but following the precedent of the Han Feizi (c. 240 BCE), Warring States period figures Shen Buhai (400–337 BCE) and Shang Yang (390–338 BCE) have commonly been taken as its "founders." Commonly thought of as the greatest of all "Legalist" texts, the Han Feizi is believed to contain the first commentaries on the Dao De Jing in history. Sun Tzu's The Art of War incorporates both a Daoist philosophy of inaction and impartiality, and a "Legalist" system of punishment and rewards, recalling political philosopher Han Fei's concepts of power and tactics. Temporarily coming to overt power as an ideology with the ascension of the Qin dynasty, the First Emperor of Qin and succeeding emperors often followed the template set by Han Fei.

Though the origins of the Chinese administrative system cannot be traced to any one person, the administrator Shen Buhai may have had more influence than any other on the construction of the merit system, and might be considered its founder, if not valuable as a rare pre-modern example of abstract theory of administration. Sinologist Herrlee G. Creel sees in Shen Buhai the "seeds of the civil service examination", and perhaps the first political scientist. Concerned largely with administrative and sociopolitical innovation, Shang Yang was a leading reformer of his time. His numerous reforms transformed the peripheral Qin state into a militarily powerful and strongly centralized kingdom. Much of "Legalism" was "the development of certain ideas" that lay behind his reforms, which would help lead to Qin's ultimate conquest of the other states of China in 221 BCE. Calling them the "theorists of the state", sinologist Jacques Gernet considered the Fajia to be the most important intellectual tradition of the fourth and third centuries BCE.

The Fajia pioneered the centralizing measures and the economic organization of the population by the state that characterized the entire period from the Qin to the Tang dynasty; the Han dynasty took over the governmental institutions of the Qin dynasty almost unchanged. Legalism rose to prominence again in the twentieth century, when reformers regarded it as a precedent for their opposition to conservative Confucian forces. As a student, Mao Zedong championed Shang Yang, and towards the end of his life hailed the anti-Confucian legalist policies of the Qin dynasty.