Studies of compact radio sources since the discovery of quasars<br />
have revealed a variety of physical properties:<br />
both in morphology and kinematics from sub-parsec to Mega-parsec scales,<br />
radiation mechanisms at frequencies from the radio to gamma-rays,<br />
theoretical models for relativistic jets, etc.<br />
The frontier discovery of VLBI observations for<br />
the compact extragalactic radio sources have triggered<br />
the extensive studies to investigate the underlying physics of<br />
the relativistic jets. In this context, the highest resolution<br />
VLBI surveys of ultra-compact radio sources provides<br />
the potentially important statistical basis for future<br />
study.In this talk, I would like to review recent VLBI surveys <br />
of compact radio sources. As one of the scientific topics for <br />
Korean VLBI Network (KVN), high-resolution VLBI survey<br />
of compact radio sources and its applications will then be discussed.