Transformers installed between the AC systems and high voltage DC systems (HVDC) enable the construction of HVDC transmission systems. These so-called converter transformers are required to provide isolation between AC and DC systems and also to enable phase shifting to realize higher pulse number converter circuits. Although the transformers are basically built on conventional, well-known high voltage, high power AC transformer technologies using well-known design know-how and manufacturing facilities, DC voltage stresses and semiconductors are used to provide the switching that enables the conversion of DC to AC or AC to DC to transfer power into or out of the AC networks. In this process DC potential is placed on the transformer’s insulation system in most HVDC converters, the switched current flows through the transformer’s windings, and the special voltage ratings needed to enable the DC power flows are described in this chapter for both line commutated converters (LCC) and voltage sourced converters (VSC). The one key requirement that makes HVDC converter transformers unique, as explained in the chapter, is that in most HVDC systems the DC voltage stresses on the insulation system are much more severe than the AC voltage stresses.