The HYSPLIT model, developed by the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory, is a comprehensive system for simulating atmospheric transport, dispersion, chemical transformation, and deposition of pollutants. It has been widely used in atmospheric science for over three decades, with more than 800 citations in scientific literature. HYSPLIT is primarily used for back-trajectory analysis to determine the origin of air masses and establish source-receptor relationships. It has also been applied to simulate the transport and dispersion of pollutants, hazardous materials, radioactive material, wildfire smoke, wind-blown dust, allergens, and volcanic ash. The model combines a Lagrangian approach for tracking individual air parcels with an Eulerian approach for computing pollutant concentrations. It has evolved significantly over the past 30 years, from estimating simple trajectories based on radiosonde data to a system that accounts for multiple interacting pollutants. HYSPLIT version 4, the basis for current model versions, includes an automated method for using multiple meteorological grids and calculates dispersion rates based on vertical diffusivity, wind shear, and horizontal deformation. It allows for different Lagrangian representations of air masses, including 3D particles, puffs, or a hybrid of both. Recent developments include the incorporation of nonlinear chemical transformation modules to simulate ozone in the lower troposphere, the use of a generalized nonlinear chemistry module to calculate the spatial and temporal distribution of photochemical species, and the inclusion of a particle-in-grid approach for air quality forecasting. HYSPLIT has also been used to estimate the transport and deposition of PCDD/F to the Great Lakes and to model the fate and transport of mercury emissions. The model has been applied to various scenarios, including the Chernobyl accident, the Rabaul volcanic eruption, and the Fukushima Daiichi nuclear power plant incident. It has also been used to simulate the transport of forest fire smoke and its effect on weather, as well as to provide forecasts for volcanic ash dispersion and to support emergency response efforts for nuclear accidents. HYSPLIT is now available for use on PC, Mac, and Linux platforms and can be run with current forecast meteorological data. It is widely used by researchers, government agencies, and emergency managers for atmospheric transport and dispersion modeling. The model is also integrated with the Real-Time Environmental Applications and Display System (READY) for real-time data and simulations. HYSPLIT continues to be a valuable tool for atmospheric science, providing critical information for environmental monitoring, public health, and emergency response.The HYSPLIT model, developed by the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory, is a comprehensive system for simulating atmospheric transport, dispersion, chemical transformation, and deposition of pollutants. It has been widely used in atmospheric science for over three decades, with more than 800 citations in scientific literature. HYSPLIT is primarily used for back-trajectory analysis to determine the origin of air masses and establish source-receptor relationships. It has also been applied to simulate the transport and dispersion of pollutants, hazardous materials, radioactive material, wildfire smoke, wind-blown dust, allergens, and volcanic ash. The model combines a Lagrangian approach for tracking individual air parcels with an Eulerian approach for computing pollutant concentrations. It has evolved significantly over the past 30 years, from estimating simple trajectories based on radiosonde data to a system that accounts for multiple interacting pollutants. HYSPLIT version 4, the basis for current model versions, includes an automated method for using multiple meteorological grids and calculates dispersion rates based on vertical diffusivity, wind shear, and horizontal deformation. It allows for different Lagrangian representations of air masses, including 3D particles, puffs, or a hybrid of both. Recent developments include the incorporation of nonlinear chemical transformation modules to simulate ozone in the lower troposphere, the use of a generalized nonlinear chemistry module to calculate the spatial and temporal distribution of photochemical species, and the inclusion of a particle-in-grid approach for air quality forecasting. HYSPLIT has also been used to estimate the transport and deposition of PCDD/F to the Great Lakes and to model the fate and transport of mercury emissions. The model has been applied to various scenarios, including the Chernobyl accident, the Rabaul volcanic eruption, and the Fukushima Daiichi nuclear power plant incident. It has also been used to simulate the transport of forest fire smoke and its effect on weather, as well as to provide forecasts for volcanic ash dispersion and to support emergency response efforts for nuclear accidents. HYSPLIT is now available for use on PC, Mac, and Linux platforms and can be run with current forecast meteorological data. It is widely used by researchers, government agencies, and emergency managers for atmospheric transport and dispersion modeling. The model is also integrated with the Real-Time Environmental Applications and Display System (READY) for real-time data and simulations. HYSPLIT continues to be a valuable tool for atmospheric science, providing critical information for environmental monitoring, public health, and emergency response.