Silicon immersion gratings have been a promising future technology for high resolution infrared spectroscopy for over 15 years. We report here on our current immersion grating research, including extensive measurements of the performance of micromachined silicon devices. We are currently producing gratings for two high resolution spectrometers: iSHELL at the University of Hawaii and IGRINS at the University of Texas and the Korea Astronomy and Space Science Institute. The gratings are R3 devices with total lengths of ~95 mm. The use of a high index material like silicon permits the spectrometers to have high resolving powers (40,000-70,000) at decent slit sizes with very small (25mm) collimated beams. The lithographic production of coarse grooves allows for instrument designs with continuous wavelength coverage over broad spectral ranges. We discuss the science requirements for grating quality and efficiency and the measurements we have made to verify that the gratings meet these requirements. The measurements include optical interferometry and measurements of the monochromatic point spread function in reflection.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.