Periodic variability in active galactic nuclei In this doctoral thesis multi frequency very long baseline interferometry observations together with multi frequency total flux density variability data of compact relativistic jets in active galactic nuclei are presented and analyzed. The main goal of the thesis is to investigate the physical mechanisms in relativistic jets responsible for such phenomena as the co existence of moving and stationary jet components jet wiggling and precession. We also aim to study the connection between the structural changes in the relativistic jets and flares in the total flux density light curves and to find observational evidences for the appearance of a primary perturbation in the base of the jet and its further propagation. In this thesis we also investigate which physical mechanisms are responsible for periodical total flux density variability and to search for periodicities as a sign of jet precession. In order to study the jet physics we used the multi frequency very long baseline interferometry technique which gives the highest possible in astronomy resolution. We also compared jet structural changes with single dish multi frequency observations spanning more than 30 years together with optical and gamma ray data. In particular analysis of the long term kinematics of two active galactic nuclei S5 1803+784 and 0605 085 shows evidence for jet precession ...