This chapter focusses on the DC side of HVDC converters, and all the aspects which have to be considered regarding the understanding of harmonic phenomena, the analysis of harmonic distortion and its effects, and the design of suitable mitigation measures where needed. Crucial to the analysis is the possible inductive interaction with parallel telecommunication systems, and a detailed description is given of the mathematical background and of practical means of analysis and derivation of suitable harmonic limits. It is recognized that as communication systems shift to digital and fibre optic communication, the risk of interference from HVDC transmission is greatly reduced, nevertheless it is still necessary to prove that this is the case, and also to design the DC side system so that abnormal stresses are not experienced due to resonances. The DC side harmonic generation of both LCC and VSC technology is discussed, leading on to modelling of the DC side circuit components and calculation of harmonic current flows. Filter design is covered, together with other possible means of reducing harmonic flow. Potential problems due to induction from nearby AC lines are analysed and mitigation measures demonstrated. Finally, methods for compliance verification and field measurements are discussed.