The switching process which is essential to the transformation of AC to DC, or vice versa, inevitably produces a waveform which deviates to some extent from a pure sine wave or pure direct current. This waveform distortion can be characterized in terms of harmonics. When specifying a new HVDC project, the topic of harmonic distortion must be considered, in terms of the distortion limits to be respected, the mitigation methods, such as filters, which may be employed, and the requirements for rating of any such apparatus. Harmonic limits, on voltage and/or current, will typically be derived from an applicable grid code, with allowance made for the pre-existing distortion and with for possible future connections. The optimum design of suitable harmonic filters depends crucially on the harmonic impedance of the AC network at the point of connection, and this must be specified in a way which allows for all possible variations of lines, loads, generation and load levels. Typically this is achieved by the use of impedance envelopes, covering all possible impedance values to be considered. If AC side filters are required, then their reactive power generation is an important aspect in determining the maximum amount of filtering that can be connected at any given HVDC load level and what is the maximum size of filtering unit which can be switched without causing an excessive voltage step. On the DC side, the main reason for limiting harmonic current is the potential for interference with nearby communication equipment, such as analogue telephone lines, due to inductive coupling. Similar inductive coupling may also occur from nearby AC power transmission lines to the DC conductors and the resulting cross-modulation of the induced fundamental frequency current may have deleterious effects on the converter transformers and may require mitigation. This chapter covers all the above aspects, focusing on how they should be treated in the Technical Specification of an HVDC project.