Blazars are defined by different observational criteria at various wavebands. But the general picture is that they are
active galactic nuclei (AGN) with a symmetric pair of relativistic jets, the approaching one pointing almost exactly
to our line of sight. The effect of relativistic beaming makes blazar jets appear very bright and compact, thus ideal
targets for very long baseline interferometry (VLBI) observations. Also, blazars are so powerful that some of them can
be detected from vast cosmological distances. Radio-loud AGN at extremely high redshifts provide a unique insight into
the AGN phenomena in the early Universe, the evolution of this class of objects on a cosmological time scale, and can
also be used as probes of cosmological models. In this talk, I briefly introduce blazars and the VLBI technique, and
present our recent observations of blazars at high redshifts (z>4) with the European VLBI Network (EVN). Surprisingly,
two of the targeted "best candidate" high-redshift blazars previously selected based on their X-ray properties and
broad-band spectral energy distribution appear anything but blazars with VLBI. I put the unexpected results in a
broader context and show the apparent diversity of compact (~1-100 milli-arcsecond scale) radio structures of AGN at
extremely high redshifts. I also touch upon some open questions concerning the earliest radio AGN of the Universe.
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