There seem to be two promising ways of investigating the evolution of radio sources: (a) the collection of data on a large number of sources so that attempts may be made to arrange them in an evolutionary sequence, (b) the study of a few sources, for which really detailed information is obtainable, to gain insight into the physical mechanisms occurring.

It is important to distinguish between the most powerful radio sources, the majority of which appear to have a simple double structure at the resolutions now attainable, and the more complex sources. The structure of the latter suggests that energetic particles have been accelerated in multiple ‘events’, each being of smaller magnitude than the events which give rise to the double sources; the total energy released may, however, be comparable.