The proposed Reflection Grating Spectrometer (RGS) on the Constellation-X mission is designed to provide high-resolution x-ray spectroscopy of astrophysical sources. Two types of reflection grating geometries have been proposed for the RGS. In-plane gratings have relatively low-density rulings (~500 lines/mm) with lines perpendicular to the plane of incidence, thus dispersing x-rays into the plane. This geometry is similar to the reflection grating spectrometer flown on the X-ray Multi-Mirror (XMM) mission. Off-plane, or conical, gratings require much higher density rulings (>5000 lines/mm) with lines parallel to the plane of incidence, thus dispersing x-rays perpendicular to the plane. Both types present unique challenges and advantages and are under intensive development. In both cases, however, grating flatness and assembly tolerances are driven by the mission's high spectral resolution goals and the relatively poor resolution of the Wolter foil optics of the Spectroscopy X-ray Telescope (SXT) that is used in conjunction with the RGS. In general, to achieve high spectral resolution, both geometries require lightweight grating substrates with arcsecond flatness and assembly tolerances. This implies sub-micron accuracy and precision which go well beyond that achieved with previous foil optic systems. Here we present a progress report of technology development for the precision shaping, assembly and metrology of the thin, flat grating substrates.