We adopt the view that the classes of active galactic nuclei (AGN) known variously as gigahertz peak spectrum (GPS) sources, compact steep-spectrum (CSS) sources, and compact symmetric objects (CSO) generally represent the same sort of object and show that both their radio spectra and optical emission can be explained by a single model which incorporates the effects produced by the interaction of a jet-driven nonthermal lobe with a dense interstellar medium. Following Begelman, we assume that these sources are young AGNs (ages less than or similar to 10(6) yr) in which the jets are propagating through an interstellar medium in which the hydrogen number density, n(H) decreases as a power law with radius, with the index delta approximate to 1-2 and n(H) similar to 10-100 cm(-3) at 1 kpc. The bow shock preceding the radio lobe is radiative at early times in such a dense environment, and the optical line emission produced by the shocked ISM and the associated photoionized precursor is proportional to the monochromatic radio power, consistent with the observational data of Gelderman & Whittle. The ionized gas surrounding the lobes has a significant emission measure and a correspondingly high free-free opacity which is responsible for the 0.1-1 GHz peaks in the radio spectra. For jet energy fluxes similar to 10(45)-10(46) ergs s(-1), consistent with the observed radio powers of these objects, the crucial observed anticorrelation between peak frequency and size is readily recovered. The form of the radio spectra (power laws at high and low frequencies) indicate that the absorption is due to a cloudy/filamentary medium with an approximately uniform distribution of opacities resulting from a combination of a two-phase interstellar medium, shock shredding of clouds impacted by the bow shock and thermal instabilities in the shocked ISM. The ionized medium enveloping the radio source also forms a Faraday screen which produces high rotation measure and substantial depolarization, readily accounting for another key property of this class of AGNs.
