Seafloor hydrothermal systems are known to play a major role in elemental exchange between ocean and crust through the interactions of circulating seawater with oceanic crust at various temperatures. It has been recognized that the seafloor hydrothermal activity significantly affects not only the ocean chemistry but also subduction zone magmatisms, mantle composition, and activity of chemolithoautotrophic microorganisms in hydrothermal vents. Therefore, elucidating the elemental behavior during hydrothermal reactions between ocean and crust is important to understand chemical evolution of ocean, crust, mantle, and life on Earth. Hydrothermally altered rock is a key product of the seafloor hydrothermal reactions, providing important information of the chemical exchange processes in the seafloor hydrothermal systems. In this paper, I summarized geochemical studies on (1) altered mid-ocean ridge basalt (MORB) from the Southwest Indian Ridge in the Indian Ocean, (2) altered greenstones from the Archean Pilbara Craton in Western Australia, and (3) altered ultramafic rocks from the Central Indian Ridge in the Indian Ocean. The results of the investigations on the Indian Ocean MORB clarified the elemental behavior between oceanic crust and circulating seawater during hydrothermal alteration of oceanic crust in modern seafloor hydrothermal systems. On the other hand, the Archean seafloor altered greenstones from the Pilbara Craton showed quite different elemental behavior during seafloor hydrothermal alteration, reflecting the difference in chemical compositions of atmosphere and ocean between modern and the Archean Earth. Moreover, studies on altered ultramafic rocks from Central Indian Ridge revealed that ultramafic rocks presented in oceanic crust as a minor component have a significant impact on hydrothermal fluid chemistry, especially H (sub 2) concentrations. This, in turn, affects biological activity at seafloor hydrothermal vents. Results of these studies on seafloor hydrothermal systems, as well as my recent investigations, portrayed geochemical relationships among ocean, crust, and life, providing important insights into co-evolution of Earth and life throughout the Earth's history.