The Romulan prototype Interphase generator
An Interphase generator is an experimental cloaking technology. It was made by combining a molecular phase inverter with a cloaking device. Matter exposed to the interphase generator would both be phased and cloaked, making it both undetectable to sensors and out of phase with normal matter, allowing the subjected matter to pass through normal matter. The Romulan Empire developed a prototype interphase generator in 2368 and tested it on a scoutship retrofitted with such a device. The test had mixed success - Romulan officer Parem was phased and cloaked but the generator failed (although Starfleet officers Geordi La Forge and Ro Laren would also be cloaked and phased), severely damaging the ship and releasing chroniton particles.
Several years prior to 2368, the Klingon Empire attempted to develop their own interphase generator technology. However, the development never got out of its preliminary stages due to several accidents. (TNG: "The Next Phase" and "The Pegasus")