Yassa (alternatively: Yasa, Yasaq, Jazag, Zasag, Mongolian: Их Засаг, Ikh Zasag) was the oral law code of the Mongols declared in public in Bukhara by Genghis Khan. It was the de facto law of the Mongol Empire, even though the "law" was kept secret and never made public. The Yassa seems to have its origin in wartime decrees, which were later codified and expanded to include cultural and lifestyle conventions. By keeping the Yassa secret, the decrees could be modified and used selectively. It is believed that the Yassa was supervised by Genghis Khan himself and his stepbrother Shikhikhutag, then the high judge (in Mongolian: улсын их заргач) of the Mongol Empire. Genghis Khan appointed his second son, Chagatai (later Chagatai Khan), to oversee the laws' execution.
 The Yasa decrees were thought to be comprehensive and specific, but no Mongolian scroll or codex has been found. There are records of excerpts among many chronicles including those of al-Maqrizi, Vardan Areveltsi, and Ibn Battuta. Moreover, copies may have been discovered in Korea as well. The absence of any physical document is historically problematic. Historians are left with secondary sources, conjecture and speculation, which describe much of the content of the overview. Historical certainty about the Yassa is weak compared to the much older Code of Hammurabi (18th century BCE) or the Edicts of Ashoka (3rd century BCE). The latter was carved for all to see on stone plinths, 12 to 15 m high, which were located throughout Ashoka's empire (now India, Nepal, Pakistan and Afghanistan). The Yassa, which is thought to be written in the Uyghur Mongolian script and written on scrolls, was preserved in secret archives and known only to and read only by the royal family. Beyond being a code of laws, the Yassa may have included philosophical, spiritual and mystical elements and so may have been thought of as a quasi-sacred or magic text.
 The exoteric aspect of Yassa outlined laws for various members of the Mongol community such as soldiers, officers and doctors. The Yassa aimed at three things: obedience to Genghis Khan, a binding together of the nomad clans and the merciless punishment of wrongdoing. It concerned itself with people, not property. Unless a man confessed, he was not judged guilty. The purpose of many decrees was probably to eliminate social and economic disputes among the Mongols and future allied peoples. Among the rules were the ban on stealing of livestock from other people, the requirement to share food with travelers, the selling of women from other families and the lack defection among soldiers. It represented a day-to-day set of rules for people under Mongol control that was strictly enforced.
 The Yassa also addressed and reflected Mongol cultural and lifestyle norms. Death via decapitation was the most common punishment unless the offender was of noble blood, when the offender would be killed by way of back-breaking, without shedding blood. Even minor offences were punishable by death. For example, a soldier would be put to death if he did not pick up something that fell from the person in front of him. Those favored by the Khan were often given preferential treatment within the system of law and were allowed several chances before they were punished.
 As Genghis Khan had set up an institution that ensured complete religious freedom, people under his rule were free to worship as they pleased if the laws of the Yassa were observed.
 Ögedei Khan, the third son of Genghis Khan and the second Great Khan, proclaimed the Great Yassa as an integral body of precedents, confirming the continuing validity of his father's commands and ordinances and added his own. Ögedei codified rules of dress,the  conduct of the kurultais, the military council. His two immediate successors followed the tradition of the Yassa.
 The Mongols who lived in various parts of the empire began to add laws that were needed in their areas.
 In the modern Turkish language (as used presently in Turkey), the word "law" is yasa, and the adjective "legal" is yasal. The word for a constitution, including the Constitution of Turkey, is Anayasa ("mother-law").
 The word yasa or Yassa exists in both Mongolic and Turkic languages. It is believed that the word comes from the Mongolian verb as- or yas- which means "to set in order". Tsereg zasakh is a phrase commonly found in old Mongolian texts like the Secret History that means "to set the soldiers in order" in the sense of rallying the soldiers before a battle. The supreme executive body of the present-day Mongolian government is called the Zasag-in gazar, which means the "place of Zasag", the "place of order". Zasag during the Qing dynasty referred to native provincial governors in Mongolia. The local office called Zasag-in gazar served as a court of the first instance and included secretaries and other officials. The verb zasaglakh means "to govern" in Mongolian. The Turkic verb yas-, which means "to spread", probably originated in Uighur Turkic and was first used by Uighur Turks.