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 > 1012 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 presentation, I give an overview of various VLBI observations carried out on both radio-loud and radio-quiet quasars at redshifts z > 4, and describe, in greater detail, the VLBI results obtained on three individual sources: the two brightest mm-wavelength quasars with z > 4, and the highest redshift radio-loud quasar known-to-date at z=6.12. Please note that this talk is similar to the one I gave at UNM in late November 2008.