GOCAD is a project devoted to the geometric modelling of 3D complex surfaces. It was developped to overcome the weaknesses of classical methods such as grid representation techniques which are for example unable to represent reverse faults. Moreover. classical CAD methods usine Bezier or BSpline interpolators are not very well adapted to fit the data corresponding to complex geological surfaces. The GOCAD geometric modelling tool is based on two new fast interpolators (DSI and DSA : J.L. Mallet 1989) minimizing a roughness criterion while respecting known data points. The GOCAD geometrical data base is specially devoted to the storage of triangulated surfaces, lines and points and it has been designed to allow interactive use and to perform rapidly operations such as intersection or union of two surfaces, intersection of a surface by a ray... Tools are provided to make automatic conversion of conventional grid models even in the presence of faults, to integrate heterogeneous data in the modelling and to allow storage of physical properties. The GOCAD project is developped in C Language under UNIX and uses X-Windows.