  The most probable initial magnetic configuration of a CME is a flux rope
consisting of twisted field lines which fill the whole volume of a dark coronal
cavity. The flux ropes can be in stable equilibrium in the coronal magnetic
field for weeks and even months, but suddenly they loose their stability and
erupt with high speed. Their transition to the unstable phase depends on the
parameters of the flux rope (i.e., total electric current, twist, mass loading
etc.), as well as on the properties of the ambient coronal magnetic field. One
of the major governing factors is the vertical gradient of the coronal magnetic
field which is estimated as decay index (n). Cold dense prominence material can
be collected in the lower parts of the helical flux tubes. Filaments are
therefore good tracers of the flux ropes in the corona, which become visible
long before the beginning of the eruption. The perspectives of the filament
eruptions and following CMEs can be estimated by the comparison of observed
filament heights with calculated decay index distributions. The present paper
reviews the formation of magnetic flux ropes, their stable and unstable phases,
eruption conditions, and also discusses their physical implications in the
solar corona.
