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The use of a gas absorption cell as the velocity metric has become a standard method of achieving the very high velocity precision necessary for detection of low-mass planetary companions to nearby stars. At present, I2, is the most commonly used gas. This gas absorption cell technique has a number of significant advantages: 1) The I2, spectrum is an absorption spectrum superimposed directly on the stellar spectrum. Thus, the reference spectrum follows exactly the same illumination pattern as the stellar spectrum. 2) The extremely narrow I2, absorption lines permit modeling of small-scale variations in the spectrograph instrumental profile. 3) It is easy and inexpensive to retrofit an existing spectrograph with an I2, cell. The I2, cell technique has been exceptionally successful in the detection of extra-solar planets. [ABSTRACT FROM AUTHOR]