This technical report gives general guidance on the subject of active filters for use in highvoltage direct current (HVDC) power transmission. It describes systems where active devices are used primarily to achieve a reduction in harmonics in the d.c. or a.c. systems. This excludes the use of automatically retuned components. The various types of circuit that can be used for active filters are described in the report@ along with their principal operational characteristics and typical applications. The overall aim is to provide guidance for purchasers to assist with the task of specifying active filters as part of HVDC converters. Passive filters are specifically excluded from this report.