A cordless telephone or portable telephone is a telephone with a wireless handset that communicates via radio waves with a base station connected to a fixed telephone line, usually within a limited range of its base station (which has the handset cradle). The base station is on the subscriber premises, and attaches to the telephone network the same way a corded telephone does.

The base station on subscriber premises is what differentiates a cordless telephone from a mobile telephone. Current cordless telephone standards, such as PHS and DECT, have blurred the once clear-cut line between cordless and mobile telephones by implementing cell handover, various advanced features, such as data-transfer and even, on a limited scale, international roaming. In these models, base stations are maintained by a commercial mobile network operator and users subscribe to the service.

Unlike a corded telephone, a cordless telephone needs mains electricity to power the base station. The cordless handset is powered by a rechargeable battery, which is charged when the handset sits in its cradle.

In the United States, seven frequency bands have been allocated by the Federal Communications Commission for uses that include cordless phones. These are:

- 1.7 MHz (1.64 MHz to 1.78 MHz & up to 5 Channels, AM System)
- 43–50 MHz (Base: 43.72-46.97MHz, Handset: 48.76-49.99MHz, allocated in 1986 for 10 channels, and later 25 Channels, FM System)
- 900 MHz (902–928 MHz) (allocated in 1990)
- 1.9 GHz (1880–1900 MHz) (used for DECT communications outside the U.S.)
- 1.9 GHz (1920-1930 MHz) (developed in 1993 and allocated U.S. in October 2005)
- 2.4 GHz (allocated in 1998)
- 5.8 GHz (allocated in 2003 due to crowding on the 2.4 GHz band).

1.7 MHz cordless phones were the earliest models available at retailers, generally identifiable by their large metal telescoping antennas. Channel selection had to be done manually by the user, and transmitted just above the AM broadcast band. These models are no longer in production, and are considered obsolete due to being very susceptible eavesdropping and to interference, especially from fluorescent lighting and automobile ignitions.

43-50 MHz cordless phones had a large install base by the early 1990s, and featured shorter bendable antennas plus auto channel selection. Due to their popularity, an over crowding of the band led to an allocation of additional frequencies, thus manufacturers were able to sell models with 25 available channels instead of just 10 channels. Despite being less susceptible to interference, these models are no longer in production and are considered obsolete because these frequencies are easily heard on practically any radio scanner. Advanced models began to use audio inversion as a basic form of scrambling to help limit unauthorized eavesdropping.

900 MHz cordless phones are still sold today and have a huge install base, features include even shorter antennas, up to 30 auto selecting channels, and higher resistance to interference. Available in three varieties; analog, digital, and digital spread spectrum, with most being sold today as budget analog models. While analog models are still susceptible to eavesdropping, scanners that have the ability to scan this spectrum can no longer be manufactured legally, but older models can still be legally purchased that are fully capable of intercepting phone calls. Digital variants can still be scanned, but are received as a digital hiss and therefor are sufficiently difficult to eavesdrop upon that personal information can be safely discussed. Because of the nature of digital, these models are also immune to static interference but can experience signal fade (brief silence) as the phone goes out of range of the base. Digital Spread Spectrum (DSS) variants spread their signal over a range of frequencies providing more resistance to signal fade. Additionally the FCC only allows DSS model phones to transmit at the full power of 1 watt, which allows increased ranger over analog and digital models.

Virtually all telephones sold in the US use the 900 MHz, 1.9 GHz, 2.4-GHz, or 5.8 GHz bands, though legacy phones can remain in use on the older bands. There is no specific requirement for any particular transmission mode on 900, 1.9, 2.4, and 5.8, but in practice, virtually all newer 900 MHz phones are inexpensive analog models with digital features such as DSSS and FHSS generally available only on the higher frequencies.

Some cordless phones advertised as 5.8 GHz actually transmit from base to phone on 5.8 GHz and transmit from phone to base on 2.4 GHz or 900 MHz, to conserve battery life inside the phone.

The recently allocated 1.9 GHz band is used by the popular DECT phone standard and is considered more secure than the other shared frequencies.