  The term active matter describes diverse systems, spanning macroscopic (e.g.
shoals of fish and flocks of birds) to microscopic scales (e.g. migrating
cells, motile bacteria and gels formed through the interaction of nanoscale
molecular motors with cytoskeletal filaments within cells). Such systems are
often idealizable in terms of collections of individual units, referred to as
active particles or self-propelled particles, which take energy from an
internal replenishable energy depot or ambient medium and transduce it into
useful work performed on the environment, in addition to dissipating a fraction
of this energy into heat. These individual units may interact both directly as
well as through disturbances propagated via the medium in which they are
immersed. Active particles can exhibit remarkable collective behaviour as a
consequence of these interactions, including non-equilibrium phase transitions
between novel dynamical phases, large fluctuations violating expectations from
the central limit theorem and substantial robustness against the disordering
effects of thermal fluctuations. In this chapter, following a brief summary of
experimental systems which may be classified as examples of active matter, I
describe some of the principles which underlie the modeling of such systems.
