Ubiquitous hydrogen plays a dominant role as the primary residual gas in low to ultrahigh vacuum, either in the form of water (unbaked systems) and/or in the form of molecular hydrogen (baked systems). Because of the very small size of the atom, hydrogen resides in interstitial sites, point, line, 2d and 3d defects in the bulk of virtually all materials and is the primary source of outgassing. At the vacuum interface, hydrogen is also located in the form of metal hydroxides. The effective outgassing rate for a given vacuum material is a function of the hydrogen surface concentration which, in turn, is a function of the density of the aforementioned sites and defects. System processing significantly affects the magnitude of the hydrogen concentration and can vary by many orders of magnitude. It is, therefore, quite difficult to analytically characterize the molecular dynamics in UHV systems without accurate knowledge of the existing concentration. In this review, the relevant parameters connected with the location and transport of hydrogen (primarily for stainless steel, but also for aluminum and other selected system materials) is presented. Gas phase variations are correlated with the surface complex chemical composition, thickness and concentration of defects. Surface diagnostics, such as AES, XPS, TDS and TOF-SIMS were employed to determine the sources of hydrogen, desorption mechanisms and the magnitude of outgassing into the vacuum space. Hydrogen solubilities and diffusivities are also presented. System processing, such as, thermal bake, glow discharge cleaning, molecular scrubbing and other outgassing reduction methods are compared. The magnitude of hydrogen outgassing is correlated with the extent of system processing.