This paper addresses the fundamentals of RF switches providing a comparison between semiconductor and RF MEMS switches. The basis of comparison is introduced by defining a figure of merit that is a function of the off-state capacitance and the on-state resistance. A simple transmission line model is presented to illustrate the impact of the switch off-state capacitance on the switch isolation and frequency range of operation. The figure of merit analysis given in this paper demonstrates that RF MEMS switches have superior insertion loss and isolation performance in comparison to MESFET and p-i-n diode switches. The paper also addresses several other design considerations beside insertion loss and isolation for selecting the right RF switch. A discussion is given on the potential use of RF MEMS switches in satellite and wireless applications.