  One characteristic feature of soft matter systems is their strong response to
external stimuli. As a consequence they are comparatively easily driven out of
their ground state and out of equilibrium, which leads to many of their
fascinating properties. Here, we review illustrative examples. This review is
structured by an increasing distance from the equilibrium ground state. On each
level, examples of increasing degree of complexity are considered. In detail,
we first consider systems that are quasi-statically tuned or switched to a new
state by applying external fields. These are common liquid crystals, liquid
crystalline elastomers, or ferrogels and magnetic elastomers. Next, we
concentrate on systems steadily driven from outside e.g. by an imposed flow
field. In our case, we review the reaction of nematic liquid crystals, of
bulk-filling periodically modulated structures such as block copolymers, and of
localized vesicular objects to an imposed shear flow. Finally, we focus on
systems that are "active" and "self-driven". Here our range spans from
idealized self-propelled point particles, via sterically interacting particles
like granular hoppers, via microswimmers such as self-phoretically driven
artificial Janus particles or biological microorganisms, via deformable
self-propelled particles like droplets, up to the collective behavior of
insects, fish, and birds. As we emphasize, similarities emerge in the features
and behavior of systems that at first glance may not necessarily appear
related. We thus hope that our overview will further stimulate the search for
basic unifying principles underlying the physics of these soft materials out of
their equilibrium ground state.
