Justus Freiherr von Liebig (12 May 1803 – 18 April 1873) was a German scientist who made major contributions to the theory, practice, and pedagogy of chemistry, as well as to agricultural and biological chemistry; he is considered one of the principal founders of organic chemistry. As a professor at the University of Giessen, he devised the modern laboratory-oriented teaching method, and for such innovations, he is regarded as one of the most outstanding chemistry teachers of all time. He has been described as the "father of the fertilizer industry" for his emphasis on nitrogen and minerals as essential plant nutrients, and his popularization of the law of the minimum, which states that plant growth is limited by the scarcest nutrient resource, rather than the total amount of resources available. He also developed a manufacturing process for beef extracts, and with his consent a company, called Liebig Extract of Meat Company, was founded to exploit the concept; it later introduced the Oxo brand beef bouillon cube. He popularized an earlier invention for condensing vapors, which came to be known as the Liebig condenser.

Justus Liebig was born in Darmstadt into the middle-class family of Johann Georg Liebig and Maria Caroline Möser in early May 1803. His father was a drysalter and hardware merchant who compounded and sold paints, varnishes, and pigments, which he developed in his own workshop. From childhood, Justus was fascinated with chemistry.

At the age of 13, Liebig lived through the year without a summer, when the majority of food crops in the Northern Hemisphere were destroyed by a volcanic winter. Germany was among the hardest-hit nations in the global famine that ensued, and the experience is said to have shaped Liebig's later work. Due in part to Liebig's innovations in fertilizers and agriculture, the 1816 famine became known as "the last great subsistence crisis in the Western world".

Liebig attended grammar school at the Ludwig-Georgs-Gymnasium in Darmstadt, from the ages of 8 to 14. Leaving without a certificate of completion, he was apprenticed for several months to the apothecary Gottfried Pirsch (1792–1870) in Heppenheim before returning home, possibly because his father could not afford to pay his indentures (a legal contract that reflects or covers a debt or purchase obligation). He worked with his father for the next two years, then attended the University of Bonn, studying under Karl Wilhelm Gottlob Kastner, his father's business associate. When Kastner moved to the University of Erlangen, Liebig followed him.

Liebig left Erlangen in March 1822, in part because of his involvement with the radical Korps Rhenania (a nationalist student organization), but also because of his hopes for more advanced chemical studies. The circumstances are clouded by possible scandal. Some scholars argue that he fled to Paris because of his involvement in radical student groups. In late October 1822, Liebig went to Paris to study on a grant obtained for him by Kastner from the Hessian government. He worked in the private laboratory of Joseph Louis Gay-Lussac and was also befriended by Alexander von Humboldt and Georges Cuvier (1769–1832). Liebig's doctorate from Erlangen was conferred on 23 June 1823, a considerable time after he left, as a result of Kastner's intervention on his behalf. Kastner pleaded that the requirement of a dissertation be waived and the degree granted in absentia.

Liebig left Paris to return to Darmstadt in April 1824. On 26 May 1824, at the age of 21 and with Humboldt's recommendation, Liebig became a professor extraordinarius at the University of Giessen. Liebig's appointment was part of an attempt to modernize the University of Giessen and attract more students. He received a small stipend, without laboratory funding or access to facilities.

The presence of existing faculty complicated his situation: Professor Wilhelm Zimmermann (1780–1825) taught general chemistry as part of the philosophy faculty, leaving medical chemistry and pharmacy to Professor Philipp Vogt in the medical faculty. Vogt was happy to support a reorganization in which pharmacy was taught by Liebig and became the responsibility of the faculty of arts, rather than the faculty of medicine. Zimmermann found himself competing unsuccessfully with Liebig for students and their lecture fees. He refused to allow Liebig to use existing space and equipment and finally committed suicide on 19 July 1825. The deaths of Zimmermann and Professor Blumhof, who taught technology and mining, opened the way for Liebig to apply for a full professorship. Liebig was appointed to the Ordentlicher chair in chemistry on 7 December 1825, receiving a considerably increased salary and a laboratory allowance.

Liebig married Henriette "Jettchen" Moldenhauer (1807–1881), the daughter of a state official, in May 1826. They had five children: Georg (1827–1903), Agnes (1828–1862), Hermann (1831–1894), Johanna (1836–1925), and Marie (1845–1920). Although Liebig was Lutheran and Jettchen Catholic, their differences in religion appear to have been resolved amicably by bringing their sons up in the Lutheran faith and their daughters as Catholics.