VERY-LONG-BASE-LINE RADIO INTERFEROMETRY SURVEYS OF THE COMPACT STRUCTURE IN ACTIVE GALACTIC NUCLEI

Very-long-baseline radio interferometry (VLBI) imaging surveys have be en undertaken since the late 1970s. The sample sizes were initially li mited to a few tens of objects but the snapshot technique has now allo wed samples containing almost 200 sources to be studied. The overwhelm ing majority of powerful compact sources are asymmetric core-jets of o ne form or another, most of which exhibit apparent superluminal motion . However 5-10% of powerful flat-spectrum sources are 100-parsec (pc)- scale compact symmetric objects; these appear to form a continuum with the 1-kpc-scale double-lobed compact steep-spectrum sources, which ma ke up 15-20% of lower frequency samples. It is likely that these sub-g alactic-size symmetric sources are the precursors to the large-scale c lassical double sources. There is a surprising peak around 90 degrees in the histogram of misalignments between the dominant source axes on parsec and kiloparsec scales; this seems to be associated with sources exhibiting a high degree of relativistic beaming, VLBI snapshot surve ys have great cosmological potential via measurements of both proper m otion and angular size vs. redshift as well as searches for gravitatio nal ''millilensing.''