The Encyclopædia Britannica has been published continuously since 1768, appearing in fifteen official editions. Several editions have been amended with multi-volume "supplements" (3rd, 5th/6th), consisted of previous editions with added supplements (10th, and 12th/13th) or undergone drastic re-organizations (15th). In recent years, digital versions of the Britannica have been developed, both online and on optical media. Since the early 1930s, the Britannica has developed several "spin-off" products to leverage its reputation as a reliable reference work and educational tool.
Encyclopedias of various types had been published since antiquity, beginning with the collected works of Aristotle and the Natural History of Pliny the Elder, the latter having 2493 articles in 37 books. Encyclopedias were published in Europe and China throughout the Middle Ages, such as the Satyricon of Martianus Minneus Felix Capella (early 5th century), the Speculum majus (Great Mirror) of Vincent of Beauvais (1250), and Encyclopedia septem tomis distincta (A Seven-Part Encyclopedia) by Johann Heinrich Alsted (1630). Most early encyclopedias did not include biographies of living people and were written in Latin, although some encyclopedias were translated into English, such as De proprietatibus rerum (On the properties of things) (1240) by Bartholomeus Anglicus. However, English-composed encyclopedias appeared in the 18th century, beginning with Lexicon technicum, or A Universal English Dictionary of Arts and Sciences by John Harris (two volumes, published 1704 and 1710, respectively), which contained articles by such contributors as Isaac Newton. Ephraim Chambers wrote a very popular two-volume Cyclopedia in 1728, which went through multiple editions and awakened publishers to the enormous profit potential of encyclopedias. Although not all encyclopedias succeeded commercially, their elements sometimes inspired future encyclopedias; for example, the failed two-volume A Universal History of Arts and Sciences of Dennis de Coetlogon (published 1745) grouped its topics into long self-contained treatises, an organization that likely inspired the "new plan" of the Britannica. The first encyclopedia to include biographies of living people was the 64-volume Grosses Universal-Lexicon (published 1732–1759) of Johann Heinrich Zedler, who argued that death alone should not render people notable.