High Voltage Direct Current (HVDC) transmission is widely used for bulk power transmission, asynchronous connection, and marine power transmission. The principle of HVDC transmission relies on AC to DC power conversion and vice versa. Generally, power electronics converters in HVDC transmission systems can be classified into three major groups: 1. power converters, 2. DC/DC converters, and 3. DC power flow controllers. Power converters convert AC power to DC power and vice versa. DC/DC converters (or named DC transformers) provide DC voltage matching; they can also be used to divide large Multi-Terminal HVDC (MT-HVDC) grids into several smaller protection zones, regulate DC voltage, isolate faults, and connect Bipolar/Monopolar configurations. DC power flow controllers (DC-PFCs) or DC current flow controller (DC-CFCs) control power flow in HVDC transmission systems, especially in MT-HVDC grids. In this chapter, power electronic converters and inverters, commonly used in HVDC transmission systems are summarized through extensive literature review, and their various topologies are introduced.