The Lagrangian particle dispersion model FLEXPART was originally designed for calculating the long-range and mesoscale dispersion of air pollutants from point sources, such as after an accident in a nuclear power plant. The model has now evolved into a comprehensive tool for atmospheric transport modelling and analysis. Its application fields are extended to a range of atmospheric transport processes for both atmospheric gases and aerosols, e.g. greenhouse gases, ozone depletion substances, short-lived climate forces like black carbon, volcanic ash and gases as well as studies of the water cycle. It supports ECMWF, NCEP, mesoscale (WRF) and climate (NorESM, CAM) wind fields in both forward and backwards modes. FLEXTPART is open source (https://www.flexpart.eu).