Anthropogenic CO2 emissions will – unless reduced – move the Baltic Sea towards a state where acidification leads to changes in species composition, potentially influencing ecosystem functioning. Model simulations indicate that acidification in the Baltic Sea generally follows the same trajectory as the open oceans, with a pH decline of 0.6 units by year 2200 in the worst-case scenario. The Baltic Sea is highly influenced by its catchment areas, which means that acidification trends are generally more complex than in the open ocean. Improved coverage of acidification monitoring is necessary to broaden the understanding of current trends, improve the capacity to predict future changes, and as an added value provide insight into productivity patterns and eutrophication trends. An indicator for acidification in the Baltic Sea is currently under development.