One of the major difficulties in meshing 3D complex geometries is to deal with non proper geometrical definitions coming from CAD systems. Typically, CAD systems don’t take care of the proper definition of the geometries for the analsys purposes. In addition, the use of standard CAD files (IGES, VDA,...) for the transfer of geometries between different systems introduce some additional difficulties. In this work, a collection of algorithms to repair and/or to improve the geometry definitions are provided. The aim of these algorithms is to make as easy as possible the generation of a mesh over complex geometries with some minimum requirements of quality and correctness. The geometrical model will be considered as composed of a set of NURBS lines and surfaces. In order to avoid human interaction, these algorithms should be able to be executed in an automatic way. The algorithms described in this paper are: collapse of entities, correction of the list of knots, reparametrization, conversion to similar cubic entity, union of curves, subdivision of curves, reorientation of the boundary of the surfaces, and collapse of small angles. With the help of these algorithms, it is possible to automate the process of importing geometry from CAD and mesh on it mantaining a minimum level of quality criteria. This process can be performed without the need of manual intervention.