RFIT2000 fits X-ray and neutron reflectivity data. The search of global minima is done via successive descent from local minima. This method can be treated as a two stage loop repeated consequently. The first stage is the local minimization with the ?2 -like criterion and the second one is the descent from the most recent local minimum. Reflectivity curve is calculated with one of the methods: Kinematic, Parratt and Matrix. Matrix method works faster for films modeled with multiple repetion of one identcal structural units. Film structure is represented with the box model. Each box is characterized with the scattering density (Re and Im), thickness and roughness of the top interface.