Gigahertz Peak Spectrum (GPS) and Compact Steep Spectrum (CSS) sources have attracted a large amount of attention in recent years since they occupy a surprisingly large fraction of the extragalactic radio source population. In this paper we summarise a theory which attempts to unify the optical emission line and radio properties of these sources. In outline the theory is as follows: The bow shock preceding the radio lobe driven into the ISM by a powerful radio jet ionizes the ISM producing both optical line emission and a free-free absorbing screen. The free-free absorption can explain the relationship between size and turnover frequency (Stanghellini et al., 1995) and the prediction of the line emission is in accord with the observation for a small sample of sources for which optical data are available.