The pH of the ocean is being lowered by its uptake of anthropogenic atmospheric carbon dioxide (CO2), produced as a result of combustion of fossil fuels and land use changes. This acidification is especially pronounced in the surface ocean, where the loadings of anthropogenic CO2 are greatest because of the direct contact with the atmosphere. Much of the work to date has tried to elucidate the biological and biogeochemical consequences of this surface ocean acidification by carrying out studies in the laboratory. This paper gives an overview of work carried out on a cruise in northwest European shelf seas in June and July 2011. The objectives of the cruise were to study ocean acidification impacts by collecting observations from the natural environment across carbonate chemistry gradients, and by carrying out short-term (96 h) bioassay CO2 perturbation experiments on natural populations. In both cases the aim was to enhance our understanding of the impacts of ocean acidification through studies of the natural world with as little artificiality as possible. Here we give an overview of the conditions encountered during this cruise and give a brief introduction to the individual studies that were carried out and whose results are presented in the separate papers in this special issue.