Many high voltage DC (HVDC) and other power electronics systems are operated and integrated into the AC system. HVDC Multi-infeed and A/DC interaction investigations are normally covered in the manufacturers’ design and engineering processes using Electromagnetic Transient (EMT) and fundamental frequency, Root-Mean-Square (RMS) simulation tools. These investigations can also be performed in a real-time simulation (RTS) environment. In case HVDC and flexible AC transmission systems (FACTS) are provided by different suppliers, RTS multi-infeed investigations cannot be performed by a single manufacturer. However, there are Transmission System Operators (TSO), utilities or authorized third party organizations that own and operate real-time laboratories equipped with relevant replicas of HVDC, FACTS, master controller or generator controls integrated in their complex AC grid models. In this case, it is an excellent alternative to cover multi-infeed and interaction testing in a RTS environment. This Chapter describes some familiar scenarios on the interaction between HVDC and AC systems, which can be categorized into the following aspects: