Abstract <p>Kink instability can be observed in the course of filament/prominence eruptions as writhing of their long axes. Instability develops when the twist angle of the field lines inside the flux rope, which constitutes the magnetic skeleton of the filament, exceeds a certain threshold, and the filament becomes activated. Although the internal twisted structures are sometimes observed in filaments/prominences and related coronal formations, it is quite difficult to reliably estimate the twist angle in them before the onset of eruption. Analysis of observations, on the other hand, shows that majority of eruptions are most likely initiated by other mechanisms: catastrophic loss of equilibrium and torus instability. However, in the late stages of eruption, helical instability can clearly manifest itself in the writhing of the initially smooth axes of eruptive prominences, turning into huge helical structures.</p>