This article discusses the formation, magnetic structure, and eruption of s
olar filaments in terms of two contrasting paradigms. The standard paradigm
is that filaments are formed by condensation of plasma on coronal magnetic
fields that are twisted or dimpled as a result of photospheric motions. Ac
cording to this paradigm, filaments erupt when photospheric motions shear t
he fields, increasing their energy and decreasing their stability. Accordin
g to a new paradigm, subsurface motions generate toroidal magnetic flux rop
es, and after these flux ropes emerge to form active regions, the most twis
ted parts migrate into the corona to form filaments. Filaments become unsta
ble and are ejected after a sufficient accumulation of twist (i.e., magneti
c helicity). Various proposed mechanisms for producing the needed helicity
are reviewed, and several observational tests are proposed to differentiate
among the possible mechanisms.