Aimsun (short for "Advanced Interactive Microscopic Simulator for Urban and Non-Urban Networks") is a software company that provides simulation software and services for transportation planning and traffic management.

Aimsun was founded in 1997 and developed Aimsun Next traffic modeling software, which simulates mobility in networks.

Aimsun also develops Aimsun Live, which is a simulation-based traffic forecasting software as well as Aimsun Auto for studying path planning in driverless vehicles, and Aimsun Ride for modeling demand-responsive transportation services.

Aimsun was acquired by Siemens in 2018 for an undisclosed sum, as part of the Siemens Mobility Intelligent Traffic Systems (ITS) unit.

In 2021, Siemens Mobility carved out the Intelligent Traffic Systems (ITS) unit, of which Aimsun is a part, and renamed it Yunex Traffic.

In 2022, Italian infrastructure group Mundys (then known as Atlantia) bought Siemens' Yunex Traffic division for 950 million euros ($1.1 billion) to expand its transport services, making Aimsun a part of the Mundys group.

Aimsun Live is a traffic forecasting software, developed and marketed by Aimsun. Traffic control centers use Aimsun Live (formerly Aimsun Online) to make real-time decisions about the management of a road network. It is used to forecast future traffic conditions based on the current state of the network and to evaluate incident response or traffic management strategies.

Aimsun Live connects with the traffic control center, continuously processing live field data. By combining these live traffic data feeds and simulations with the emulation of congestion mitigation strategies, Aimsun Live can accurately forecast the future network flow patterns that will result from a particular traffic management or information provision strategy. Aimsun Live was launched in 2008 and is now fully deployed on Interstate 15 in San Diego, Grand Lyon in France, and other locations worldwide.