The HERMES experiment uses the 27.5 GeV beam of longitudinally polarized electrons of the HERA accelerator together with an internal gas target to study the spin structure of the nucleon. The ability to identify hadrons in coincidence with the scatteredelectron is crucial for the determination of the contributions of the different quark flavors to the nucleon spin. For this purpose a Ring Imaging CHerenkov (RICH) detector was developed. It is able to identify pions, kaons and protons over the relevant momentum range of 2–15 GeV/c by using two radiators, silica aerogel and
gas. This detector was installed in May 1998 in the HERMES experiment. It is the first RICH detector in an actual experiment that uses aerogel as a radiator.