Gasses tend to have low absorption coefficients in the millimeter wavelength region; absorption cells with path lengths of hundreds of meters are needed for millimeter wave gas-phase spectroscopy. Three types of long-path cell are discussed here: tuned cavities, untuned cavities, and optical multiple-pass cells. The operating principles of each type are described, along with the advantages and limitations of each type when used in the millimeter wavelength region. Several examples of each type of cell are given. An optical analysis of a three-mirror optical multiple-pass cell is performed, for the purpose of optimizing this cell for millimeter wave spectroscopy, with the result that a cell with mirrors one meter in diameter can give a path length of 500 meters while conserving the power from a presently available black body source.