We discuss Hubble Space T elescope (HST ) WFPC2 observations through the broad red Ðlter F702W of 30 3CR sources from the Compact Steep Spectrum (CSS) radio source sample, and present 11 new HST /WFPC2 images through linear ramp Ðlters (LRF), isolating either rest-frame [O II] 3727 or A [O III] 5007 radiation. In nearly all the CSS galaxies, we Ðnd high surface brightness emission that is A aligned with the radio axis. The strong nuclear PSF prevents us from detecting such aligned light at

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... ar levels in most of the CSS quasars. However, a comparison between CSS galaxies and quasars with PSF signatures removed reveals no inconsistency with the viewing angle uniÐcation scheme. The alignment e †ect in CSS sources is not a strong function of redshift, and is seen over the entire redshift range of the sample Our analysis of the LRF images and complementary . troscopy reveals that the aligned light is predominantly emission-line gas. These observations demonstrate the existence of dense gas in the host galaxies of CSS sources, strongly interacting with the expanding radio sources. Assuming such a shock interaction, cooling-time arguments suggest that lobe expansion speeds of km s~1 and ambient densities of D1 cm~3 are consistent with the observed Z1000 o †set between the leading edge of the radio lobe and the optical line emission.