Integrated Services Digital Network (ISDN) is a network technology that supports the digital transmission of simultaneous voice and data traffic, along with support for video and fax, over the public switched telephone network. ISDN became popular worldwide in the 1990s, but has largely been replaced by modern long-distance network technologies.

While telecommunications companies gradually converted their telephone infrastructures from analog to digital, the connections to individual homes and businesses—the so-called "last mile" network—remained on old signaling standards and copper wire. ISDN was designed as a way to migrate the transition to digital signaling. Businesses found ISDN especially valuable because of the increased number of desk phones and fax machines their networks had to support.

Many people learned about ISDN as an alternative to traditional dial-up Internet access. Although the cost of residential ISDN Internet service was relatively high, some consumers were willing to pay more for a service that advertised up to 128 Kbps connection speeds compared to the 56 Kbps (or slower) speeds of dial-up connections.

Connecting to ISDN Internet required a digital modem instead of a traditional dial-up modem, plus a service contract with an ISDN service provider. Eventually, the much faster network speeds supported by newer broadband Internet technologies such as DSL caused most customers to abandon ISDN.