We have successfully developed High Temperature Superconducting (HTS) MicroElectroMechanical (MEM) switches for RF applications. A typical switch is composed of a superconducting Yttrium Barium Copper Oxide (YBa 2Cu3O7) coplanar waveguide structure with a gold bridge membrane suspended above an area of the center conductor covered with Barium Titinate (BaTiO3) ferroelectric. A control voltage applied between the membrane and transmission line causes the membrane to collapse on top of the dielectric layer by electrostatically induced force; this in turn allows the RF signal to capacitively shunt to ground. Initial testing of switches showed very promising RF behavior with insertion losses less than 0.1 dB with 30 dB isolation at 3 GHz. These switches rely on the "ON/OFF" impedance ratio to achieve switching; this is determined by the dielectric constant of the BaTiO3 ferroelectric. These switches will be operated at cryogenic temperatures; we have investigated the impedance behavior vs. temperature of the HTS/ferroelectric/metal heterostructure to better understand the behavior of the insulating layer at cryogenic temperatures. © 2005 IEEE.