Small and young radio sources

It is currently accepted that compact and bright radio sources characterized by a convex spectrum peaking at frequencies ranging from 100 MHz to a few GHz are young objects. In this scenario, high frequency peaker (HFP) radio sources, with a turnover frequency higher than 5 GHz are good candidates to be extremely young radio sources with ages of up to a few thousand years. The knowledge of the conditions in young radio source is fundamental in order to draw reliable evolution models able to describe the entire life-cycle of the radio emission. Given the high spatial resolution and the large frequency range spanned, VLBI observations provide a unique opportunity to constrain the physical conditions in young radio sources, and to investigate the role played by the environment on the source growth. Recent VLBI results are discussed here.