We present some results concerning the possibility of determining the structure of solar active regions using measurements of the vector magnetic field on the Sun surface as boundary conditions for the new numerical extrapolation codes. From these computations the main features of these configurations, as shear and twist (which are particular forms of magnetic helicity), are then used as ingredients to define model problems and solved for the magnetohydrodynamic (MHD) analysis of solar eruptive phenomena, in which ejection (or redistribution) of helicity occurs.