By combining the signal from two telescopes in an interferometer, it is possible to measure small angular motions on the sky very precisely, down to tens of micro-arcseconds. This project will employ the astrometric search technique with the newly-commissioned PRIMA interferometer to detect the small motion of a star caused by an unseen companion, ranging from stellar companions to planetary companions. In addition to detecting the presence of a planet, it is also possible to measure the planet mass. For students interested in instrumentation experience, this project also has the possibility of working with the instrument development at the ESO observatory in Chile or the ESO headquarters in Germany.