An overview on the use of grisms from high refractive index optical materials is given. When using grisms manufactured from silicon or germanium two IR focal-reducers of the European Southern Observatory (ESO) can serve as medium resolution echelle spectrometers. A silicon echelle grism allowing for a spectral resolution of 5000 for a 1 arcsec slit is being developed for SOFI, a near IR instrument featuring a 1024 Rockwell HgCdTe detector at ESO's 3.5m New Technology Telescope. For TIMMI2, ESO's new 10/20 micrometers instrument for the 3.6m telescope a germanium echelle grism is being built. For TIMMI a 10 micrometers camera featuring a 64 X 64 detector a low resolution germanium grism yielding a spectral resolution (lambda) /(Delta) (lambda) equals 200 for a 1 arcsec slit has already been successfully commissioned. The manufacturing process, the status and performances will be presented. Moreover we show some astronomical results.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.