ATSAS is a program suite for small angle scattering (SAS) data analysis and modelling. Small angle scattering of X-rays (SAXS) and neutrons (SANS) is widely used to study the structure of biological macromolecules in solution and non-biological isotropic and partially ordered objects (inorganic, colloidal solutions, polymers in solution and bulk). ATSAS covers all the analysis steps from SAS data reduction, evaluation of the geometrical parameters and pair distance distribution functions, ab initio shape reconstruction and atomistic modelling. The ATSAS suite includes modelling programs that work with monodisperse solutions, oligomeric mixtures, semi-flexible systems and intrinsically disordered proteins. Many programs can be invoked from the interactive graphical user interface of the program PRIMUS. ATSAS also includes a number of command-line tools which are useful for automation of processing of large amounts of experimental data.