AM: Foundation for personal entertainment and national news prior to WWII and upon intro of television began losing ground.

FM: Primarily responsible for saving radio because of a stronger signal, better sound, installation in the automobile. FM radio was not a growing market until GM decided to put the radio in autos.

Satellite Radio: Only a decade old and has not gained profitability or market share. However, the portability has increased its popularity. Sirius and XM are trying to merge to survive the internet growth of both radio, live audio, and total world wide use without cost.

Internet: Has been around longer than satellite radio and is currently exceedingly popular in Europe. In fact there are more Internet radio stations world wide than all other radio formats combined. Major reason: Low cost of reception, lack of government regulation, and ability to use on personal computers without proprietary hardware. The portability factor, which has now become available will cause the medium to explode.