This paper describes the interferometer systems on the Large Helical Device (LHD). LHD is equipped with five interferometer systems, each of which has a different operational purpose and measurable electron density range. A single-channel millimeter-wave interferometer is mainly used for low-density plasmas along a horizontal line of sight on the equatorial plane. Wavelengths of 1 and 2 mm are used for vibration compensation based on two-color interferometry, which has been used since the first operation of LHD. A 13-channel CH 3 OH laser interferometer (wavelength of 119 mm) covers almost the whole poloidal cross sections of LHD plasmas with a chord separation of 90 mm. It routinely provides temporal behavior and profiles of the electron density. The laser has been developed as a collaboration between the National Institute for Fusion Science (NIFS) and Chubu University. An 80-channel CO 2 laser interferometer (10.6 mm) is employed for high-density plasmas such as superdense core plasmas. It adopts an imaging technique with three slablike beams and array detectors to measure the density profile precisely. A phase contrast imaging interferometer, which measures density fluctuations, is combined with the CO 2 laser interferometer. Since LHD has strong magnetic shear, a distribution of the density fluctuations is evaluated by using shear technique. A conventional millimeter-wave (4 mm) interferometer is also installed at a divertor region to measure dynamic density responses in a divertor leg. The phase counter used on these interferometers was originally developed at NIFS. The phase resolution of a typical phase counter is 1/100 fringe with a temporal response of 10 ms.