THERMOS is a user-friendly, free and open-source software to make heat network planning faster, more efficient, and more cost effective. The software was developed by a team of planning experts & practitioners from universities, local and city-wide authorities, energy and environmental agencies, and specialist consultancies based in the UK, Spain, Poland, Latvia, Denmark, Germany, Portugal, and Romania as part of the THERMOS (Thermal Energy Resource Modelling and Optimisation System) EU Horizon 2020 funded research project.