cD galaxies are supergiant elliptical galaxies found generally in the central parts of rich clusters, which have an extended halo-like component (envelope) in addition to the underlying de Vaucouleurs-Sérsic elliptical galaxy-like component. This envelope can extend to radial distances of > 500 kpc (Oemler 1976, Schombert 1988). There have been many theories to explain the formation of these envelopes. These include tidal stripping, where material is stripped from neighbouring galaxies; mergers and fusions, where the envelope is built up hierarchically by successive mergers with large and small galaxies; primordial origin, where the envelope is formed at the same time as the rest of the elliptical galaxy (which appears to be related to theories of early formation of the largest galaxies); and cooling flows: in clusters with X-ray emission there is often a minimum temperature in the centre interpreted as a flow of cooling gas towards the centre of the cluster, where the gas can cool sufficiently, forming stars. The colours of the stars in the envelopes will be affected by their process of formation and subsequent evolution.