This thesis has presented an in-depth study of the propagation of harmonic voltages in a meshed transmission system with a large proportion of underground cables. The background for this research was based on specific problems observed in connection with the introduction of underground cables into the Danish transmission system, where cable laying has resulted in both unexplained increases in measured harmonic distortions and unexpected scattering throughout the meshed transmission grid. This research has begun by first examining the harmonic problem in relation to a meshed grid, then through propagation studies for standing harmonic voltage waves, and finally with the identification of effective mitigation locations in the grid. This chapter concludes the research by answering the objectives and presenting the main contributions and future work.