  Solar prominences and filaments (prominences projected against the solar
disk) exhibit a large variety of fine structures which are well observed down
to the resolution limit of ground-based telescopes. We describe the
morphological aspects of these fine structures which basically depend on the
type of a prominence (quiescent or active-region). Then we review current
theoretical scenarios which are aimed at explaining the nature of these
structures. In particular we discuss in detail the relative roles of magnetic
pressure and gas pressure (i.e., the value of the plasma-beta), as well as the
dynamical aspects of the fine structures. Special attention is paid to recent
numerical simulations which include a complex magnetic topology, energy balance
(heating and cooling processes), as well as the multidimensional radiative
transfer. Finally, we also show how new ground-based and space observations can
reveal various physical aspects of the fine structures including their
prominence-corona transition regions in relation to the orientation of the
magnetic field.
