There are many pieces of the puzzle still needed to clarify the properties of the GPS and CSS sources and their place in the powerful radio source paradigm.
1. Fainter samples of sources should be studied to fill in the P-l plane.
2. The polarization properties and Faraday RMs of the sources need to be determined with high spatial resolution.
3. High dynamic range radio images of GPS sources (especially quasars) need to be obtained to determine the true range of morphology and to locate the cores.
4. Deep multicolor optical imaging of the hosts and environments of GPS and CSS and comparison samples of large-scale radio sources is needed to confirm whether the host galaxies and clustering environments are similar in these objects.
5. Sensitive near-far-IR observations should be obtained to determine the bolometric luminosity and IR colors in order to constrain the stellar populations, the AGN, and its circumnuclear environment.
6. Searches for cold gas via CO and 21 cm HI are needed to determine the gas content of the host galaxies and determine whether some objects could be "frustrated."
7. Sensitive high spatial resolution and high dispersion optical spectra are needed to study the kinematics of the emission-line gas and determine its relationship to the radio source. Combined with broadband colors and polarimetry, the spectra will also elucidate the nature of the aligned component in CSS sources.
8. X-ray observations of complete samples of sources should be obtained in order to determine the strength of the central AGN, the columns of any obscuring material, and the nature of the clustering environments.
Much theoretical work is needed on how radio sources should evolve including the evolution of the relativistic particles, magnetic fields, and efficiency.
I am grateful to many colleagues who provided reprints, preprints, and information in advance of publication; to my collaborators, especially Stefi Baum and Carlo Stanghellini; and to Wim de Vries for permission to present the results of work in progress. Thanks are due to Stefi Baum, Wim de Vries, Greg Taylor, Anton Koekemoer, Carla and Roberto Fanti, and Carlo Stanghellini for comments on the manuscript. Wim de Vries helped with the figures. I would like to thank Harvey Butcher and the Netherlands Foundation for Research in Astronomy for hospitality during which this review was begun. This research made use of (1) the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and (2) NASA's Astrophysics Data System Abstract Service.