instant messaging (IM), form of text-based communication in which two people participate in a conversation over their computers or smartphones via an Internet-based chat room or application.

Instant messaging (IM) platforms, generally referred to as messaging services or apps, often monitor for users’ online presence and show an indication of their availability. For example, the popular workplace messaging service Slack shows a green circle next to a user’s profile name if they are available to chat. IM software relies on a central server or servers to monitor user availability. In the past, when a user logged into an IM system, the login was recognized by the system, and other online users who had that address listed as a “buddy,” or friend, were notified of the user’s presence. With the modernization of such apps, users rarely have to log in or out of an application to receive messages, and applications rarely send notifications to friends simply to indicate online availability. IM software establishes a direct connection between users so they can talk to each other synchronously, in real time. The technology came to the forefront of digital communication in the 1990s in response to ongoing battles between commercial ventures engaged in its development.

One of the precursors to a formal IM was the Compatible Time-Sharing System (CTSS), which originated in 1961 at the Computation Center at the Massachusetts Institute of Technology (MIT). CTSS was housed in a large mainframe. Users connected to the mainframe through remote dial-up terminals to send messages back and forth to one another and share files. CTSS soon grew beyond MIT, allowing several hundred users from a number of colleges to converse with one another by 1965, thereby adopting modern IM-like qualities.

IM was invented in 1971 as a chat function on a government computer network. American computer scientist Murray Turoff created IM as part of the Emergency Management Information Systems and Reference Index (EMISARI) for the Office of Emergency Preparedness. Its original purpose was to help exchange information which would aid the U.S. government during emergencies. One of EMISARI’s first uses was to facilitate communication among government officials to assist the anti-inflation wage and price control efforts of the Nixon administration. EMISARI users accessed the system through teletypewriter terminals linked to a central computer. EMISARI continued to be used by the U.S. government for management of emergency situations until 1986. The EMISARI chat function was called the Party Line and was originally developed to replace telephone conferences. Party Line users all had to log on to the same computer over phone lines and read the text of the chats on Teletype units.

During the 1970s the first public chat software emerged. “Talk,” designed to work within the UNIX operating system, also required that users be logged on to the same computer to use the program. This was truly the forerunner of IM systems, since users could send a message to anyone else on the system and a note would pop up on the user’s terminal. This software was often used in combination with “Finger,” a program that allowed users to determine whether one user or another was present online at the time.

The first large-scale rollout of IM came from America Online (AOL). IM had been a part of the AOL browser as early as 1988, in the form of lists of acquaintances that let AOL customers know when their friends, relatives, or other acquaintances who also used AOL were online. Such lists were called “buddy lists” after the rollout of AOL Instant Messenger (AIM) in 1997. AIM flourished and, as the popularity of the Internet grew, so did the demand for software systems that allowed real-time conversation. The late 1980s also saw the introduction of Internet Relay Chat (IRC) software for group conversations, and by the mid-1990s other IM software, such as ICQ (or “I Seek You”) for non-AOL Internet users, also became available.

By the early 2000s several IM systems were in use on the Internet, with multiple versions for different computer operating systems (Windows, Mac OS, Linux). Such systems included Apple’s iChat, which made its debut in 2002 during the rollout of OS X Jaguar, the third version of Apple’s Mac OS X operating system. Beginning in 2011, Apple products used the centralized service iMessage (via the Apple Messaging application) to facilitate communication between users. Though Apple’s Messaging app supports MMS and SMS messages, which are delivered using cell service, the app uses the Internet to send “iMessages.” Skype, an IM and video conferencing service that was introduced in 2003, was popular for its messaging and its video-call-oriented framework. Microsoft bought Skype in 2011, but the application was soon overshadowed by options such as Zoom and Microsoft Teams, both of which gained popularity during the COVID-19 pandemic. Google Talk was first linked to the company’s Gmail service in 2005. The service has since gone through multiple iterations, with its name changed to “Hangouts” and “Allo,” among others. In 2024 Google owned the services Google Meet and Google Chat, with Google Chat designated as the company’s official IM service.

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IM became linked to social media platforms with the release of MySpaceIM by MySpace in 2006, Facebook’s Facebook Chat in 2008, and Facebook Messenger in 2011. Many more platforms have integrated IM chatting as part of their features. WhatsApp, owned by Meta Platforms, which also owns Facebook, is prominent in the space, reporting more than 2 billion monthly users. Slack and Microsoft Teams are workplace-oriented platforms that have modernized IM features. With the proliferation of artificial intelligence, chatbots have become popular as well, and users sometimes choose to message a bot rather than a human.