Predictions on growing and decay of geomagnetic storms from solar wind condi- tions follow on improving. Nowadays, it is possible to reproduce roughly the hourly variation of the index Dst from solar wind data by several techniques (linear and non-linear). The problem arises when a storm is made of several substorms nearly in time. Then, the main phase of Dst index is not properly reproduced by any technique. Moreover, the relative importance of storms and substorms in building of RC is still an open problem.