 From 1956 to 1986, the "Big Three" national commercial networks were ABC, CBS, and NBC (with a few limited attempts to challenge them, such as National Telefilm Associates [and its NTA Film Network] and the Overmyer Network).From 1954 to 1970, National Educational Television was the national clearinghouse for public TV programming; the Public Broadcasting Service (PBS) succeeded it in 1970.Today, more than fifty national free-to-air networks exist.