Seafloor hydrothermal systems play a significantly important role in Earth's energy and geochemical budgets and support the existence and development of complex biological ecosystems by providing nutrient and energy to microbial and macrafaunal ecosystems through geochemical fluxes. Heat output and fluid flow are key parameters which characterize hydrothermal systems at oceanic spreading centers by constraining models of hydrothermal circulation. Quantification of heat flux at discrete sources from direct measurements is particularly essential for examining the partitioning of heat flow into focused and diffuse components of venting and determining geochemical fluxes from these two modes of flow.This book is fundamentally focused on three main topics: (1) Design and development of direct heat flow measuring devices for hydrothermal systems, (2) Collection of new heat output results on four cruises between 2008 and 2010 along Mid-ocean Ridges (MORs) to estimate total heat output, (3) Deformation and uplift associated with serpentinization at MORs and subduction zones.