  We review observations of extended regions of radio emission in clusters;
these include diffuse emission in `relics', and the large central regions
commonly referred to as `halos'. The spectral observations, as well as Faraday
rotation measurements of background and cluster radio sources, provide the main
evidence for large-scale intracluster magnetic fields and significant densities
of relativistic electrons. Implications from these observations on acceleration
mechanisms of these electrons are reviewed, including turbulent and shock
acceleration, and also the origin of some of the electrons in collisions of
relativistic protons by ambient protons in the (thermal) gas. Improved
knowledge of non-thermal phenomena in clusters requires more extensive and
detailed radio measurements; we briefly review prospects for future
observations.
