MQTT (MQ Telemetry Transport or Message Queue Telemetry Transport) is an ISO standard (ISO/IEC PRF 20922) publish-subscribe-based "lightweight" messaging protocol for use on top of the TCP/IP protocol. It is designed for connections with remote locations where a "small code footprint" is required or the network bandwidth is limited. The publish-subscribe messaging pattern requires a message broker. The broker is responsible for distributing messages to interested clients based on the topic of a message. Andy Stanford-Clark and Arlen Nipper of Cirrus Link Solutions authored the first version of the protocol in 1999.
The specification does not specify the meaning of "small code footprint" or the meaning of "limited network bandwidth". Thus, the protocol's availability for use depends on the context. In 2013, IBM submitted MQTT v3.1 to the OASIS specification body with a charter that ensured only minor changes to the specification could be accepted. MQTT-SN  is a variation of the main protocol aimed at embedded devices on non-TCP/IP networks, such as ZigBee.
MQTT defines methods (sometimes referred to as verbs) to indicate the desired action to be performed on the identified resource. What this resource represents, whether pre-existing data or data that is generated dynamically, depends on the implementation of the server. Often, the resource corresponds to a file or the output of an executable residing on the server.
- Waits for a connection to be established with the server.
- Waits for the MQTT client to finish any work it must do, and for the TCP/IP session to disconnect.
- Waits for completion of the Subscribe or UnSubscribe method.
- Requests the server unsubscribe the client from one or more topics.
- Returns immediately to the application thread after passing the request to the MQTT client.
MQTT is designed to support wireless networks with varying levels of latency due to occasional bandwidth constraints or unreliable connections. There are several projects that implement MQTT.
- Facebook Messenger. Facebook has used aspects of MQTT in Facebook Messenger for online chat. However, it is unclear how much of MQTT is used or for what.
- IECC Scalable DeltaRail's latest version of their IECC Signaling Control System uses MQTT for communications within the various parts of the system and other components of the signaling system. It provides the underlying communications framework for a system that is compliant with the CENELEC standards for safety-critical communications.
- The EVRYTHNG IoT platform uses MQTT as an M2M protocol for millions of connected products.
- On October 8, 2015, Amazon Web Services announced Amazon IoT based on MQTT.
- The Open Geospatial Consortium SensorThings API standard specification has a MQTT extension in the standard as an additional message protocol binding. It was demonstrated in a US Department of Homeland Security IoT Pilot.
- The OpenStack Upstream Infrastructure's services are connected by an MQTT unified message bus.
- In 2015, Adafruit launched a free MQTT cloud service for IoT experimenters called Adafruit IO. 
- MQTT 3.1.1 specification
- "OASIS Message Queuing Telemetry Transport (MQTT) Technical Committee". OASIS. Retrieved 9 May 2014.
- Andy, Stanford-Clark. "MQTT For Sensor Networks (MQTT-SN) Protocol Specification Version 1.2" (PDF). Retrieved 9 May 2014.
- IBM WebSphere MQ Date accessed 2013-11-18
- Choosing Your Messaging Protocol: AMQP, MQTT, or STOMP | VMware vFabric Blog - VMware Blogs. Blogs.vmware.com (2013-02-19). Retrieved 2013-10-23.
- "Constrained Application Protocol (CoAP) RFC 7252". The Internet Engineering Task Force (IETF). Retrieved 15 November 2015.
- "InternetOfThings". XMPP WIKI. Retrieved 9 May 2014.
- "Internet Protocols for the Smart Grid RFC 6272". IETF. Retrieved 9 May 2014.
- "Building Facebook Messenger". Retrieved 15 October 2015.
- "AWS IoT – Cloud Services for Connected Devices". Retrieved 21 October 2015.
- "S&T's Internet of Things Pilot Demonstrates 'State of the Practical'". Retrieved 31 March 2016.
- "OpenStack Infra Firehose". Retrieved 30 August 2016.
- "Coming Soon: Adafruit IO". Retrieved 29 March 2017.