GADGET is a code for collisionless and gasdynamical cosmological simulations. It combines N-body methods for collisionless particles with smoothed particle hydrodynamics (SPH) for gas. The code has a serial version and a parallel version for supercomputers. It uses a tree algorithm for gravitational forces, or GRAPE hardware for faster calculations. It supports periodic boundary conditions via Ewald summation and allows adaptive timesteps for all particles. GADGET can simulate large-scale structure formation, galaxy clusters, and galaxy interactions. It uses individual and adaptive timesteps, and dynamic tree updates to handle large dynamic ranges. The code has been used in simulations with up to 7.5×10^7 particles. It includes SPH for gas dynamics and handles both collisionless and collisional systems. The code is publicly available and has been tested for accuracy and efficiency. It uses a combination of tree algorithms and GRAPE hardware for gravitational force calculations. The code also includes a time integration scheme with adaptive timesteps and handles both collisionless and gas dynamics. It is suitable for cosmological simulations and has been used in studies of dark matter, galaxy formation, and the intergalactic medium. The code is flexible and efficient, allowing a wide range of applications in cosmology.GADGET is a code for collisionless and gasdynamical cosmological simulations. It combines N-body methods for collisionless particles with smoothed particle hydrodynamics (SPH) for gas. The code has a serial version and a parallel version for supercomputers. It uses a tree algorithm for gravitational forces, or GRAPE hardware for faster calculations. It supports periodic boundary conditions via Ewald summation and allows adaptive timesteps for all particles. GADGET can simulate large-scale structure formation, galaxy clusters, and galaxy interactions. It uses individual and adaptive timesteps, and dynamic tree updates to handle large dynamic ranges. The code has been used in simulations with up to 7.5×10^7 particles. It includes SPH for gas dynamics and handles both collisionless and collisional systems. The code is publicly available and has been tested for accuracy and efficiency. It uses a combination of tree algorithms and GRAPE hardware for gravitational force calculations. The code also includes a time integration scheme with adaptive timesteps and handles both collisionless and gas dynamics. It is suitable for cosmological simulations and has been used in studies of dark matter, galaxy formation, and the intergalactic medium. The code is flexible and efficient, allowing a wide range of applications in cosmology.