Assessments of HVDC converter station equipment and system losses are covered in this chapter. The chapter covers the two major types of HVDC systems, which are line commutated converters (LCC) and voltage sourced converters (VSC). These two types of converters have different loss characteristics. It is important to know in detail how and where losses are generated in the various pieces of equipment installed in HVDC converter stations to achieve a full understanding of where the losses are generated. There are several reasons to determine the losses. (1) It affects the rating of the equipment since the temperature increases due to current flowing and accordingly the equipment should be designed for the current rating/temperature rise. (2) Loss estimates are used to evaluate the cost of losses over the life of the HVDC system. (3) The loss estimates are also used to compare the overall losses in a planned station between systems to be built using different converter technologies possibly delivered by different suppliers with different loss profiles. The maximum, worst-case losses should be used when the system is designed. However, considering the uncertainties involved in estimating the losses and the assumptions used to estimate the cost of the losses over the life of the HVDC system, it might be appropriate to use a weighted average of the power dissipation expected over the life of the installed HVDC system instead of detailed seasonal or annual loss estimates. The losses in the AC system connected to a converter station and losses in the DC side connected to the converter station such as lines or cables are not covered in this chapter.