Among all softwares used in the SAXS data analysis, GENFIT is one of the most (if not the one!) complete. It has more them 60 theoretical models inside it, allowing the user to select among several biologically relevant systems, like proteins, micelles, vesicles, bilayers, and so on... SAS curves calculated from a theoretical model can be smeared to allow for the instrumental resolution. The user can fit the experimental data selecting one or more models from a list including more than 30 models, starting from simple asymptotic behaviours (Guinier's law, Porod's law, etc.) down to complete atomic structures. Besides, the software also allows the user to use the Global fitting procedure, where several Scattering curves can be analysed concomitantly, being also possible to link different parameter from different scattering curves. Some models, which are defined in terms of both form and structure factors, take into account the interactions between scattering particles in solution. GENFIT is able to simultaneously fit more SAS curves via a unique model or a mixture of models (the global fitting procedure). In the latter case, some specific model parameters can be shared by any selection of the experimental curves. Model parameters can be related to the experimental chemical-physical conditions (temperature, pressure, concentration, pH, etc.) by means of link functions, which can be freely defined by the user. On the other hand, GENFIT can be used to generate theoretical SAS curves from a given model and/or from the knowledge of the species in solution. It can hence be a useful instrument to find the optimum experimental conditions for a planned SAS experiment. GENFIT is written in Fortran. Versions 2.0 and higher make use of a graphical user interface (GUI) to manage input files and execute the calculations. The software was mainly developed by prof. Francesco Spinozzi, from Universita Politecnica delle Marche