The aims of the Aegean Hydrothermal Fluxes and Biological Production project were to estimate the fluxes of fluids, chemicals, heat and bacteria from hydrothermal vents, establish the controls on venting dynamics, measure the productivity in the region of the vents and establish the effect of the vents on biodiversity of both prokaryotes and eukaryotes. This paper presents an initial synthesis of the project results. Research was done both by land-based SCUBA diving and from several vessels at a number of active sites in the near-shore coastal regions of Miles and Kos, with some additional studies at Methana, Lesbos and Santorini. Vent water composition showed very large variations. This was due to the mixing, of hydrothermal reservoir fluids, vapour condensate and seawater altered by interactions of fluid-sediment-bacteria in different proportions, in the gasohydrothermal vents. The composition ranged from nearly sea water with only slightly reduced pH, to higher or lower salinity fluids with a pH as low as 3 and with large enrichments in heavy and trace metals. Phase separation was a common feature at these shallow vents. The dry gas phase was mainly CO2, but with significant amounts of H2S, CH4 and H-2. These fluids commonly passed through soft sediments before venting from the seafloor and induced a convection cell of pore-water entrainment from deeper sediment layers into the water column with a consequent `re-charge' down-flow of seawater into the sediment around the vent outlets. Such complex conditions may well explain the high biodiversity of Bacteria, Archaea and epifaunal species surrounding the vents. As many as 44 % of the archaeal lineages detected were found to represent novel phyla. Epifaunal diversity was particularly high with over 200 species recorded at the shallower Miles vents. These vents may form a `stepping-stone' for warmer water species to colonise the surrounding areas when water temperatures permit. (C) 2000 Elsevier Science Ltd. All rights reserved.