A crystal radio receiver, also called a crystal set, is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source and runs on the power received from radio waves by a long wire antenna. It gets its name from its most important component, known as a crystal detector, originally made with a piece of crystalline mineral such as galena. This component is now called a diode.

Crystal radios are the simplest type of radio receiver, and can be handmade with a few inexpensive parts, like an antenna wire, tuning coil of copper wire, crystal detector and earphones. They are technically distinct in many respects from ordinary radios because they are passive receivers, while other radios use a separate source of electric power such as a battery or the mains power to amplify (magnify) the weak radio signal from the antenna so it is louder. Thus crystal sets produce rather weak sound and must be listened to with earphones, and can only pick up stations within a limited range. Developed from about 1895 to 1906 by pioneer radio researchers Karl Ferdinand Braun, Jagadish Chandra Bose and others, crystal radios were the first widely used type of radio receiver, and the main type used during the wireless telegraphy era. At the end of that era, around 1920, they were superseded by the first amplifying receivers, which used vacuum tubes (Audions), and became obsolete for commercial use. However they continued to be built by hobbyists and youth groups such as the Boy Scouts as a way of learning about the technology of radio. Today they are still sold as educational devices, and there are groups of enthusiasts devoted to their construction who hold competitions comparing the performance of their home-built designs.