Modern hydrothermal vents provide diverse environments for microorganisms. Here there is a large phylogenetic and physiological diversity of bacteria and archaea, occurring in a wide range habitats. An assumption is that similar communities of microorganisms have been present on Earth for an extremely long time, given that there is direct evidence of marine hydrothermal activity going back to the Archaean eon (which began 4 billion years ago), and the hypothesis that life may have originated in these environments. In this presentation I will review the fossil record of microorganisms at hydrothermal vents, focussing on volcanogenic massive sulfides (VMS), which formed at high temperature vents, and jaspers (iron-silica rocks), which formed at low-temperature, sulfide-poor vents. Occurrences of microorganisms in VMS go back to the Paleoarchean and in jaspers to the Eoarchaean (3.770, or possibly 4.280, billion years ago), with the latter being the possibly the oldest organisms yet discovered on Earth. These very early dates suggest that life may have been possible on Mars during its equivalent aged warmer period, and that life may be found at putative hydrothermal sites on the icy moons with liquid oceans (e.g. Europa and Enceladus).