The igneous oceanic crust is a vast potential habitat for microorganisms, and thus, part of the marine deep biosphere. By comparison to sediment in the deep biosphere, life in igneous oceanic crust is relatively unexplored and unknown to science. However, the pore fluid volume in igneous oceanic crust could represent about 10 times larger potentially habitable space for colonization by microorganisms by comparison to sediment. The igneous oceanic crust is also hydrologically active, with the entire fluid volume of the oceanic basins circulating through ridge flanks about every 200,000 years—relatively rapid on geologic timescales. Here, we review recent microbiological studies that have been conducted in igneous oceanic crust, starting with analysis of seafloor rocks and minerals, moving to deeper crustal samples collected through the recent phase of the ocean drilling program, and concluding with in situ microbiological experiments conducted with Circulation Obviation Retrofit Kit subseafloor observatories. The chapter includes the current best estimates of the size of the marine deep biosphere harbored in igneous oceanic crust, and highlights future research directions that are anticipated in the next phase of the ocean drilling program.