It is only eight years ago that scientists gathered at UNESCO headquarters in Paris for the 1st Symposium on the Ocean in a High-CO₂ World. The term ocean acidification was hardly used at that time and few scientists had investigated its possible impacts on marine life. By the end of that meeting it was clear that uptake of anthropogenic CO₂ by the ocean and the associated changes in ocean chemistry have the potential to endanger marine organisms and ecosystems ocean-wide. A remarkable scientific endeavour has occurred since the Paris symposium in 2004. With barely 20 publications per year on possible impacts of CO₂-induced changes in ocean chemistry in 2004, this number has increased to 130 per year at the time of the 2ndSymposium on the Ocean in a High-CO₂ World in Monaco in 2008 and is expected to pass 300 publications per year in 2012. Ocean acidification and the associated threat for marine life are now widely recognized, not only in the scientific community, but increasingly in the general public and are starting to be acknowledged among policy makers. This special issue intends to provide an outlet for the most recent results in ocean acidification research presented at the 3rd Symposium on the Ocean in a High-CO₂ World in Monterey, September 24-27, 2012. All symposium participants are invited to submit their original scientific work presented at the symposium and within the scope of Biogeosciences to this special issue.