By the year 2005, the ocean had taken up ca 40% of total CO2 emissions to the atmosphere originating from human activities since the onset of the industrial era. The uptake of CO2 by the ocean drives major changes in seawater carbonate chemistry. The weak acid CO2 reacts with water thereby increasing the concentrations of bicarbonate ions and protons. A fraction of CO2 is neutralized by reaction with carbonate ions which decreases the saturation state of seawater with respect to calcium carbonate minerals. Changes in seawater carbonate chemistry are reported from time series stations throughout the world ocean. Model simulations suggest that the average pH of the surface ocean has already dropped by 0.1 units and is projected to decrease further by 0.3 to 0.4 units up to 2100. A large effort is devoted to ocean acidification research and the emerging picture is one of great complexity. A variety of organisms build skeletal structures of calcium carbonate and a decrease in saturation state might threaten their ability to form and maintain them. On the other hand, the increase in total dissolved inorganic carbon associated with the uptake of CO2 might turn out to be beneficial for primary producers with a rather inefficient carbon acquisition pathway. After a brief introduction to the chemistry of ocean acidification, I present an overview of the present understanding of biological impacts. I address experimental studies at the organism scale, move up to the population level and finally discuss impact evaluation at the ecosystem level.