A device that can convert a digital bit stream into an analogue signal suitable for transmission over some analogue communication channel (modulation), and can convert incoming analogue signals back into digital signals (demodulation). Modems are used to connect digital devices across analogue transmission lines. Most modems are designed to match specific national or international standards so that data communication equipment from one manufacturer can talk to that of another.

Modems can be packaged in many ways: as add-in cards or PC cards allowing personal computers to communicate over ordinary phone lines, as small external units, or as rack-mounted sets for large applications requiring many simultaneous connections. Modern modems, for attachment to a normal telephone line, can handle data at 56 000 bps, which is about the upper limit attainable over voice-frequency channels. Broadband modems allow users to connect at speeds of several gigabits per second using ADSL or similar services.