The thesis at hand presents the results of multi-frequency Very Long
Baseline Interferometry (VLBI) observations of jets associated with active galactic nuclei. The results are discussed in the context of broad-band jet emission models and used to constrain physical properties of the parsec-scale core -- inner part of the jet which is likely related to the observed high-energy emission. The analysis of the frequency-dependent core position shift supports interpretation of the parsec-core as the integral part of the jet; the observed core position is determined by opacity. Synchrotron self-absorption is found to be the dominating opacity mechanism in the observed sources. A comparison between radio sources possessing jets aligned closely to the line of sight with those aligned at large angles to it supports the idea that the bright extragalactic gamma-ray sources are associated with jets affected by relativistic beaming
