This article provides an overview of RF design challenges in 5G/6G satellite–terrestrial integrated systems to overcome to realize ubiquitous connectivity. Interference mitigation, antenna arrays, multiple input/multiple output (MIMO) technologies, power amplification, and novel RF architectures for future 6G systems all potentially work to overcome the RF hurdles of satellite-terrestrial integrated systems. Integration solutions include passive RF technologies such as filters and active RF technologies such as wideband-front end modules and 5G new radio (NR) over satellite networks. Finally, system-level integration solutions are examined including new RF payload architectures and reconfigurable intelligent surfaces (RISs).