Microwave limb sounding is powerful to measure atmospheric compositions, temperature and pressure with high vertical resolution. During the past few decades many countries have developed several Microwave limb sounders to improve our understanding of stratospheric ozone chemistry and dynamics, the interaction of composition and climate and pollution in the upper troposphere. This issue will outline five existed MLS instrument and analyze their adopted techniques for lower noise and higher sensitivity as well as their capabilities comparisons. In American, the Jet Propulsion Laboratory (JPL) has already developed three MLS instruments: the UARS-MLS, EOS-MLS and CAMEO-SMLS which is ready for launch in 2011. In Japan, its JEM/SMILES is the first MLS instrument to use superconductor-insulator-superconductor (SIS) mixers with a mechanical 4-K refrigerator in space. In Sweden, the millimeter and sub millimeter limb-emission sounder on Odin uses actively cooled Schottky receivers with auto-correlators and wide hand AOS.