The author will review the phenomenology of water adsorption/desorption in vacuum systems. A review of the literature of outgassing shows that for unbaked metals, the outgassing is dominated by water, and the outgassing rate (Q) obeys a power law of the form Q = Q0 t -a, where a is near unity. A series of outgassing measurements@footnote 1@ have been performed on well characterized stainless steel surfaces which show that the outgassing power law exponent can vary from a = 0.7 - a = 1.2 as the metal surface is exposed from extremely dry N2 to increasing partial pressures of H2O. Relatively simple engineering formulae exist which quantify the amount of adsorbed/desorbed H2O as a function of exposure pressure, time and temperature. Models have been developed consistent with these data which invoke the oxide layer as the source volume for the outgassing and assume that the outgassing rate is limited by diffusion from the near surface region. Alternative models for outgassing of water have been described which assume surface desorption as the only source term.@footnote 2@ The author will discuss the additional measurements and modeling that are needed to achieve a more sophisticated understanding of water outgassing from metal surfaces. This work supported by US DOE Contract No. DE. @FootnoteText@ @footnote (1)@ Minxu Li and H.F. Dylla, J. Vac. Sci. Technol. A11, 1702 (1993); A12, 1772 (1994); A13, 1872 (1995). @footnote (2)@ P.A. Redhead, J. Vac. Sci. Technol. A, 13, 467 (1995).