This CIGRE Green Book on High Voltage Direct Current (HVDC) Transmission Systems is intended to assist electrical engineers and power system planners and others in understanding how to select, apply, and manage power electronic systems used for conversion of AC to DC and vice versa and how to integrate HVDC links in AC systems. This chapter deals with the levels of conducted and radiated electromagnetic interference emanating from the converters caused by the switching of the converter valves. These valves are typically placed in large halls inside buildings with air insulation that provides a clean environment, which prevents exposure of the valves from rain, dust, snow etc. that would impact the insulation properties for the valves.