AOL Radio powered by Slacker, (formerly AOL Radio powered by CBS Radio, and prior AOL Radio featuring XM), is an online radio service available in the United States only.

AOL Radio can trace its roots to two companies it acquired on June 1, 1999, for $400 million: Spinner.com and Nullsoft. Spinner.com was formerly known as TheDJ.com. Nullsoft was the maker of the popular Winamp and SHOUTcast products. Both new organizations operated out of the same office in San Francisco. The Spinner.com brand was retired in July 2003, but exists today as an AOL Music blog and a series of channels on AOL Radio.

AOL Radio launched as Radio@AOL, essentially a rebranded Spinner.com, using technology from Real Networks on October 16, 2001 as part of the AOL 7.0 software announced that same day.

In its first month of operation, AOL reported that 2.2 million members accessed Radio@AOL, making it one of AOL's most popular features.

Initially, Radio@AOL was available only to AOL Members. On May 22, 2002, AOL released the free Radio@Netscape for non-members as part of the new Netscape 7.0 browser. On August 22, 2002, AOL released Radio@Netscape Plus. Beginning in 2004, AOL started metering Radio@Netscape to allow only two hours of usage per day. AOL did this to avoid paying copyright royalties and to encourage users to become AOL Members.

AOL Inc. (NYSE: AOL, previously known as America Online, written as "AOL" and stylized as "Aol." but commonly pronounced as an initialism) is an American global Internet services and media company. AOL is headquartered at 770 Broadway in New York. Founded in 1983 as Control Video Corporation, it has franchised its services to companies in several nations around the world or set up international versions of its services. AOL is headquartered in New York City, but has many offices throughout cities in North America, such as Atlanta, Baltimore, Beverly Hills, Boston, Chicago, Detroit, Dulles, Mountain View, Orlando, San Francisco, and Toronto. London and Tokyo are its foreign offices. As of December 2011, it serves 3.3 million paid subscribers.^{[citation needed]}

AOL is best known for its online software suite, also called AOL, that allowed customers to access the world's largest "walled garden" online community and eventually reach out to the Internet as a whole. At its prime, AOL's membership was over 30 million members worldwide, most of whom accessed the AOL service through the AOL software suite. The company has claimed that it had 34 million subscribers at its peak, although an undisclosed portion were using free hours.

Radio is the transmission of signals through free space by electromagnetic waves with frequencies significantly below visible light, in the radio frequency range, from about 3 kHz to 300 GHz. These waves are called radio waves. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.

Information, such as sound, is carried by systematically changing (modulating) some property of the radiated waves, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.

The etymology of "radio" or "radiotelegraphy" reveals that it was called "wireless telegraphy," which was shortened to "wireless" in Britain. The prefix radio- in the sense of wireless transmission, was first recorded in the word radioconductor, a description provided by the French physicist Édouard Branly in 1897. It is based on the verb to radiate (in Latin "radius" means "spoke of a wheel, beam of light, ray").