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Compact steep-spectrum (CSS) and peaked spectrum (PS) radio sources arecompact, powerful radio sources. The multi-frequency observational propertiesand current theories are reviewed with emphasis on developments since theearlier review of O'Dea (1998). There are three main hypotheses for the natureof PS and CSS sources. (1) The PS sources might be very young radio galaxieswhich will evolve into CSS sources on their way to becoming large radiogalaxies. (2) The PS and CSS sources might be compact because they are confined(and enhanced in radio power) by interaction with dense gas in theirenvironments. (3) Alternately, the PS sources might be transient orintermittent sources. Each of these hypotheses may apply to individual objects.The relative number in each population will have significant implications forthe radio galaxy paradigm. Proper motion studies over long time baselines havehelped determine hotspot speeds for over three dozen sources and establish thatthese are young objects. Multifrequency polarization observations havedemonstrated that many CSS/PS sources are embedded in a dense interstellarmedium and vigorously interacting with it. The detection of emission line gasaligned with the radio source, and blue-shifted HI absorption and [OIII]emission lines indicates that AGN feedback is present in these objects --possibly driven by the radio source. CSS/PS sources with evidence of episodicAGN over a large range of time-scales have been discussed. The review closeswith a discussion of open questions and prospects for the future.

Compact steep-spectrum and peaked-spectrum radio sources