Root beer is a beverage made from a variety of plant roots and spices. Recipes for root beer abound, so different commercially sold brands or homemade versions may differ widely in taste. Root beer is typically non-alcoholic, though alcoholic versions were more common in the past, before the prohibition era. Sassafras root is usually the main flavoring of root beer, which takes its name from this ingredient. Imitation sassafras is most commonly used today, as natural sassafras is mildly carcinogenic.

The earliest incarnations of root beer were slightly alcoholic and used as medicine for mouth complaints, including sore throat, cough, and canker sores. This type of root beer was brewed from roots or root extracts and sold at drug stores. Philadelphia pharmacist Charles Elmer Hires is credited with the invention of root beer as we currently know it. In 1866, at the age of 15, he began selling Hires Root Beer as a powder that was mixed with water, sugar, and yeast to make a carbonated soft drink. In 1893, he began selling bottled, ready to drink root beer.

Hires marketed his drink as a flavorful and healthy alternative to alcohol in the early days of the temperance movement, and his efforts were quite successful. The prohibition against alcohol in the United States during the 1920s cemented the popularity of root beer, and many breweries that had formerly produced beer began manufacturing root beer instead. Root beer also rose to popularity in England in the late 19th and early 20th centuries, with many local varieties, using roots other than sassafras, in existence.

Root beer, being an alternative to alcoholic beverages, is often served to children. Some commercial root beers, such as Barq's, include caffeine, but most do not. A popular dessert using root beer is the root beer float, which adds vanilla ice cream to the beverage and is traditionally eaten with a long spoon. A&W Restaurants, which offer their own root beer on tap, are well-known for their floats.

Root beer declined in popularity after the discovery that sassafras is carcinogenic in the 1960s, with cola largely taking its place. However, pure sassafras is no longer used in commercial root beers. Root beer still boasts many fans, at least in the United States, and the adventurous have been known to brew their own root beer concoctions.