TV DX and FM DX is the active search for distant radio or television stations received during unusual atmospheric conditions. The term DX is an old telegraphic term meaning "long distance."

VHF/UHF television and radio signals are normally limited to a maximum "deep fringe" reception service area of approximately 40–100 miles (64–161 km) in areas where the broadcast spectrum is congested, and about 50 percent farther in the absence of interference. However, providing favourable atmospheric conditions are present, television and radio signals sometimes can be received hundreds or even thousands of miles outside their intended coverage area. These signals are often received using a large outdoor antenna system connected to a sensitive TV or FM receiver, although this may not always be the case. Many times smaller antennas and receivers, such as those in vehicles, will receive stations farther than normal depending on how favourable conditions are.

While only a limited number of local stations can normally be received at satisfactory signal strengths in any given area, tuning into other channels may reveal weaker signals from adjacent areas. More consistently strong signals, especially those accentuated by unusual atmospheric conditions, can be achieved by improving the antenna system. The development of interest in TV-FM DX as a hobby has grown over time, with enthusiasts installing and upgrading HF/UHF antennae for the purpose of gaining a higher range. The TV-FM DX hobby is similar to other radio/electronic related hobbies such as amateur radio, Medium Wave DX, or short-wave radio; and organisations such as the Worldwide TV-FM DX Association have developed to coordinate and foster the further study and enjoyment of VHF/UHF television and FM broadcast DX.

After the Alexandra Palace, London 405-line BBC channel B1 TV service was introduced in 1936, it soon became apparent that television could be received well outside the original intended service area.

For example, in November 1938, engineers at the RCA Research Station, Riverhead, Long Island, accidentally received a 3,000-mile (4,800 km) transatlantic F2 broadcast of the London 45.0 MHz, 405-line BBC Television service.

The flickering black-and-white footage (characteristic of F2 propagation) included Jasmine Bligh, one of the original BBC announcers, and a brief shot of Elizabeth Cowell, who also shared announcing duties with Jasmine, an excerpt from an unknown period costume drama and the BBC's station identification logo transmitted at the beginning and end of the day's programmes.

This reception was recorded on 16 mm movie film, and is now considered to be the only surviving example of pre-war, live British television.

The BBC temporarily ceased transmissions on September 1, 1939 as World War II began. After the BBC Television Service recommenced in 1946, distant reception reports were received from various parts of the world, including Italy, South Africa, India, the Middle East, North America and the Caribbean.