Fajia (Chinese: 法家; pinyin: fǎjiā), or the School of fa (incl. law, method), often translated Legalism, was a bibliographic school of primarily Warring States period classical Chinese philosophy, including more administrative works traditionally said to be rooted in Huang-Lao Daoism. Addressing practical governance challenges of the unstable feudal system, their ideas 'contributed greatly to the formation of the Chinese empire' and bureaucracy, advocating concepts including rule by law, sophisticated administrative technique, and ideas of state and sovereign power. They are often interpreted in the West along realist lines. Though persisting, the Qin to Tang were more characterized by the 'centralizing tendencies' of their traditions.

The school incorporates the more legalistic ideas of Li Kui and Shang Yang, and more administrative Shen Buhai and Shen Dao, with Shen Buhai, Shen Dao, and Han Fei traditionally said by Sima Qian to be rooted in Huang-Lao (Daoism). Shen Dao may have been a significant early influence for Daoism and administration. These earlier currents were synthesized in the Han Feizi, including some of the earliest commentaries on the Daoist text Daodejing. The later Han dynasty considered Guan Zhong to be a forefather of the school, with the Guanzi added later. Later dynasties regarded Xun Kuang as a teacher of Han Fei and Qin Chancellor Li Si, as attested by Sima Qian, approvingly included during the 1970s along with figures like Zhang Binglin.

With a lasting influence on Chinese law, Shang Yang's reforms transformed Qin from a peripheral power into a strongly centralized, militarily powerful kingdom, ultimately unifying China in 221 BCE. While Chinese administration cannot be traced to a single source, Shen Buhai's ideas significantly contributed to the meritocratic system later adopted by the Han dynasty. Sun Tzu's Art of War recalls the Han Feizi's concepts of power, technique, wu wei inaction, impartiality, punishment, and reward. With an impact beyond the Qin dynasty, despite a harsh reception in later times, succeeding emperors and reformers often recalled the templates set by Han Fei, Shen Buhai and Shang Yang, resurfacing as features of Chinese governance even as later dynasties officially embraced Confucianism.

Later applied to figures like Shang Yang and Han Fei, Fajia or "fa school" had no figures associated with it in the earlier Shiji Records of the Grand Historian. Sima Tan coined it as a flawed category in an argument for a syncretic political Daoism, as supposedly adopting the best elements of the schools. Han scholar Liu Xiang (77–6 BCE) and later used it as a category of Masters Texts in the imperial library.

It would become a major category in Han dynasty catalogues, namely the Han state's own Book of Han (Hanshu, 111ce), Journal of Literature Chapter 30. Alongside the Book of Lord Shang, Shen Buhai, Shen Dao and the Han Feizi, six other texts were listed under it, now lost, including Li Kui and Han minister Chao Cuo (Hanshu CH1), leaving four unidentifiable works. With Guan Zhong only officially associated later, the Hanshu considered Li Kui the first of broader works on regulations, influencing Shang Yang.

Apart from Shang Yang, the division of texts into schools most notably sorts works that can themselves be associated with a concept of syncretic political "Daoism" in the Shiji, differing from Daoism as later understood. With the rise of Confucianism, what Tan called the Dao school (Daojia "Daoism") was redefined as rejecting "ritual learning, and abandoning humanity and duty, saying that the employment of purity and vacuity alone can be used to rule."

A confused conflation of Fajia with a Shang Yangian Legalism would theoretically develop from a pairing of Shang Yang and Shen Buhai in the Han Feizi. While Sima Tan conceived Fajia as extending to all, it was philosophically more inclusive than law, pertaining to office divisions. In Shen Buhai and the Han Feizi, these are not primarily legal in origin.

Although described as strict, Sima Tan (and evidently Liu Xiang) "clearly" understood that standards (fa) were used in the administration, and not just harsh penal law like Shang Yang. They were not however all considered strict until they were associated with Tan's fa school. The Han Feizi advocates well defined, mechanically strict administration (Xing-Ming) at the end of the Warring States period. Baring more resemblance to Han dynasty level conceptual thinking, the Han Feizi's level of strict mechanical functioning isn't evident earlier, including by its predecessor Shen Buhai.