The MDI (Michelson Doppler Imager) was developed by the Solar Oscillations Investigation to investigate the properties of: the solar interior using the tools of helioseismology; and, the photosphere and corona using more conventional techniques. The helioseismology study requires a long series of uninterrupted observations that are free of atmospheric distortions and not contaminated by Earth's rotation. The MDI operates in a variety of modes to produce solar maps in very narrow wavelength bands. The wavelength passband is scanned across a spectral line by tuning a pair of Michelson interferometers. Full-disk or 3.2x magnified 1024 x 1024 pixel solar images are produced with 4 or 1.2 arcsec resolution. The line-of-sight velocity, line, and continuum intensities, line-of-sight magnetic field, and a simpler but less accurate magnetic field proxy can all be computed from filtergrams on board. The solar limb position and transverse velocity are determined on the ground by analyzing continuum images. At high bit rate (160 Kbps), two complete compressed images can be recovered each minute. Low data rate (5 Kbps) will be used during 56% of the time. The time required to obtain an image in one observable ranges from 24 s for the Doppler parallel velocity and line intensity to 60 s for the oscillation mode amplitudes (computed on-board). The noise level and sensitivity for the Doppler velocity are 21 m/s and <1 m/s, respectively. These quantities are 0.1% and 0.01% for line intensity; 0.3% and 0.01% for continuum intensity; 6 G and <0.5 G for parallel magnetic field; 0.02 arcsec and 0.001 arcsec for limb position; 2 cm/s and <1 cm/s for oscillation mode amplitudes; 0.4% and 0.01% for the magnetic proxy; and 15 m/s and TBD for the transverse velocity. The instrument uses a series of increasingly narrow wavelength filters centered on the 676.8 nm Ni I spectral line, and a CCD camera. The stabilized beam enters a controlled oven containing two fixed filters and two tunable Michelson interferometers with 0.02 and 0.01 nm bandpasses. Rotating two quarter wave plates to specified angles determines the wavelength of the filtergram. Four observing programs are defined. The Structure Program uses the 5 Kbps rate, and returns highly processed mode amplitudes plus other averages. This program operates at all times and uses the camera for at least 30 s of each minute (10 filtergrams). The Dynamics Program operates during the 60 consecutive days each year when continuous high-rate telemetry coverage is available. It transmits a full disk velocity image and either a full disk intensity or high resolution velocity image each minute. Campaign programs will be conducted during the remaining ten months of the year when the high-rate telemetry is provided for eight hours each day. The remaining 30 s of each minute can be used for various other investigations. Data from the Magnetic Field Program should be available in near-real time at the SOHO Experiment Operations Facility. This direct high-rate data channel will be provided for about five minutes at the end of each tape recorder dump (three times per day). Magnetograms will nominally be made nine times each day, and will be available in real time to the SOHO experiments. This information is from the paper ``The Solar Oscillations Investigation - Michelson Doppler Imager,'' by J. T. Hoeksma et al. (Proceedings of the First SOHO Workshop, ESA SP-348, pp. 9-12, November 1992).