MQTT is a lightweight publish/subscribe messaging transport protocol. The OASIS specification describes it as a Client Server publish/subscribe messaging transport protocol that is lightweight, open, simple, and designed to be easy to implement. These properties make it well suited to machine-to-machine (M2M) and Internet of Things (IoT) settings, where devices may have very little memory and processing power and operate over unreliable or expensive network links.
The protocol was invented in 1999 by Andy Stanford-Clark of IBM and Arlen Nipper of Arcom (later Eurotech), originally to monitor oil pipelines over satellite connections. The constraints of that environment (minimize bytes on the wire, tolerate intermittent connectivity, and run on small embedded controllers) shaped the design, and they explain why MQTT has aged into one of the dominant protocols for sensor networks and connected devices decades later.
MQTT works through a central broker. Clients do not talk to each other directly; instead a publisher sends a message tagged with a topic, and the broker delivers it to every client that has subscribed to that topic. Topics are hierarchical strings such as building/floor2/temperature, and subscribers can use wildcards to match whole subtrees. The protocol defines three Quality of Service levels (at most once, at least once, and exactly once), letting an application trade delivery guarantees against overhead. Features such as retained messages and the last-will-and-testament, which notifies subscribers when a client disconnects abnormally, round out the design while keeping the protocol small. MQTT runs over TCP/IP.
MQTT moved into open standardization through OASIS. Version 3.1.1 became the first OASIS Standard version of MQTT, published on 29 October 2014, and it was also adopted as ISO/IEC 20922 in 2016. Version 5.0, published on 7 March 2019, added enhancements including improved scalability, richer error reporting, formal request/response support, user properties for extensibility, and performance optimizations for small clients, while preserving the lightweight character that made the protocol popular.
Because the protocol is small, royalty-free, and well specified, MQTT became a default choice for IoT platforms, home automation, telemetry, and mobile messaging backends. Major cloud providers expose MQTT endpoints for device connectivity, and open-source brokers and client libraries exist for nearly every language and embedded platform, cementing its role as a foundational protocol for connecting large fleets of constrained devices.