At millimeter and submillimeter wavelengths, pressure broadened molecular spectral lines make the atmosphere a natural limitation to the sensitivity and resolution of astronomical observations. Tropospheric water vapor is the principal culprit. The translucent atmosphere both decreases the signal, by attenuating incoming radiation, and increases the noise, by radiating thermally. Furthermore, inhomogeneities in the water vapor distribution cause variations in the electrical path length through the atmosphere. These variations result in phase errors that degrade the sensitivity and resolution of images made with both interferometers and filled aperture telescopes. To evaluate possible sites for the Millimeter Array, NRAO has carried out an extensive testing campaign. At a candidate site at 5000 m altitude near Cerro Chajnantor in northern Chile, we deployed an autonomous suite of instruments in 1995 April. These include a 225 GHz tipping radiometer that measures atmospheric transparency and temporal emission fluctuations and a 12 GHz interferometer that measures atmospheric phase fluctuations. A sub-millimeter tipping photometer to measure the atmospheric transparency at 350 micrometer wavelength and a submillimeter Fourier transform spectrometer have recently been added. Similar instruments have been deployed at other sites, notably Mauna Kea, Hawaii, and the South Pole, by NRAO and other groups. These measurements indicate Chajnantor is an excellent site for millimeter and submillimeter wavelength astronomy. The 225 GHz transparency is better than on Mauna Kea. The median 225 GHz transparencies measured at Chajnantor and at the South Pole are comparable.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.