In recent years, various optical surveys revealed large samples of quasars
out to z of 6. Studies have shown that at such high redshifts we are approaching the Epoch of Reionization, when the first stars and
massive black holes were formed. High redshift quasars provide both
interesting constraints on the growth of the first supermassive black
holes, and light sources with which to probe the ionization history of
the Universe. While observations at radio wavelengths show that only a
few of these high-z quasars are radio-loud, (sub)mm studies reveal a
significant fraction (20-30%) of these most distant sources to be
copious emitters of far-IR radiation with FIR luminosities > 10^12 Lsun.

VLBI, with its unprecedented resolving power, can be utilized to study
the highest redshift quasars to 1) look at their physical structures at
milliarcsecond resolution, 2) test for strong gravitational lensing to
address the high mass values of their supermassive black holes, and 3)
test, by direct imaging, whether the dominant power source at radio
frequencies is an AGN or starburst, particularly in those sources that
are strong FIR emitters.

In this talk I will give an overview of the Very Long Baseline Array (VLBA) and the High Sensitivity Array (HSA), and present observational results obtained on various radio-loud and radio-quiet quasars at redshifts z > 4 with the VLBA and the HSA. I will also provide a summary on a similar work that targets sum-mm galaxies at z~2.