In this chapter we discuss hydrothermal and metasomatic processes that have taken place in impact structures, subsequent to the collapse of the transient cavity and the cooling of the melt sheet and melt rocks. Most of what follows is drawn from Pirajno et al. (Aust J Earth Sci 50:775-796, 2003), Pirajno (Aust J Earth Sci 52:587-620, 2005) and Pirajno and Van Kranendonk (Aust J Earth Sci 52:329-352, 2005), particularly for the Australian examples. The flow of hot aqueous solutions commonly results in the formation of mineral deposits. Therefore, knowledge of post-impact hydrothermal activity is important because it may have resulted in economic mineral deposits. The world-class and widely known Sudbury mineral deposits (Ni, Cu, PGE, Pb, Zn, Au) are perhaps the best and most celebrated expression of mineralization directly related to a meteorite impact (Lightfoot, Nickel sulfide ores and impact melts -origin of the Sudbury Igneous Complex. Elsevier, Amsterdam, 662pp, 2016). Several lines of evidence suggest that the giant gold deposits of the Witwatersrand in South Africa may have been reworked or even enhanced by the effects of the large Vredefort impact structure. These cases will be examined briefly in the sections that follow. Hydrothermal circulation systems associated with impact events have been reported from the Ries (Germany), Puchezh-Katunki (Russia), Jamtland (Sweden), Roter Kamm (Namibia), Manson (USA), the above-mentioned Vredefort, Kardla (Estonia), Sudbury and Haughton (Canada) structures (Newsom et al., J Geophys Res 91:E239-E251, 1986; Koeberl et al., Geoch Cosmo Acta 53:2113-2118, 1989; Naumov, Meteoritics 28:408-409, 1993; Sturkel et al., Eur J Miner 10:589-609, 1998; Ames et al., Geology 26: 447-450, 1998; McCarville and Crossey, Geol Soc Am Sp Pap 302:347-379, 1996); Grieve and Thierriault, Annu Rev Earth Planet Sci 28:305-338, 2000; Osinski et al., Meteor Planet Sci 36:731-745, 2001; Molnar et al., Econ Geol 96:1645-1670, 2001; Puura et al., Impact-induced replacement of plagioclase by K-feldspar in granitoids and amphibolites at the Kardla crater, Estonia. In: Gilmour I, Koeberl C (eds) Impacts and the early earth. Springer-Verlag, Berlin, pp 417-445, 2000 and Geochemistry of K-enriched impactites, based on drillings into the Kardla Crater, Estonia. Geol Soc Am Abs with Programs, Denver, Oct. 2002, p 341, 2002). Recently, aspects of hydrothermal alteration in the yChicxulub impact structure have been published in Meteoritic and Space Science (Luders and Rickers, Meteor Planet Sci 39:1187-1198, 2004; Zurcher and Kring, Meteor Planet Sci 39:1199-1222, 2004; Goto et al., Meteor Planet Sci 39:1233-1247, 2004).