In this chapter, we evaluate how the incorporation of H 2 O as a thermodynamic component influences phase relations in a peridotite composition. This component – present either in the form of hydrous minerals, aqueous fluids and hydrous melts, or as a structurally-bonded trace element at defect sites of nominally anhydrous minerals (NAMs) – may influence upper-mantle rheology in diverse ways. By presenting various natural cases, we identify key incorporation mechanisms and assess their role in the microstructural evolution of ultramafic rocks at different depths in the Earth’s interior. These data suggest that the influence of either aqueous fluids or hydrous melts on rheology out-matches that of NAMs or stable hydrous phases across much of the lithospheric mantle. Consequently, future research is expected to shift towards a better understanding of the transient conditions in the lithosphere that control the availability and transport of aqueous fluids and hydrous melts. These transient conditions are likely to play a more dominant role than the sole ability of hydrous defects in NAMs – a role that is currently less well-constrained experimentally – in controlling the ductile deformation of the upper mantle.