From the foreword: This report, CBD Technical Series No. 75, “An updated synthesis of the impacts of ocean acidification on marine biodiversity”, represents an enormous scientific effort by researchers and experts from around the world to synthe- size the best available and most up-to-date information on the impacts of changing ocean pH on the health of the world’s oceans. Among other findings, the report notes that ocean acidifica- tion has increased by around 26% since pre-industrial times and that, based on historical evidence, recovery from such changes in ocean pH can take many thousands of years. The report outlines how ocean acidification impacts the physi- ology, sensory systems and behavior of marine organisms, and undermines ecosystem health. It, furthermore, shows that impacts due to ocean acidification are already under- way in some areas and that future projected impacts could have drastic irreversible impacts on marine ecosystems. Despite the growing body of information on ocean acidifica- tion, the report points out key knowledge gaps and, in light of the many complex interactions related to ocean chemis- try, stresses the difficulty of assessing how future changes to ocean pH will affect marine ecosystems, food webs and ecosystems, and the goods and services they provide. This report, which presents complex scientific information on ocean acidification in a clear and understandable way, provides an important reference point for scientists, policy- makers and anyone else interested in understanding how ocean acidification affects our oceans and the vital services they provide. As the need for urgent action to address ocean acidification becomes ever more pressing, collaboration among governments and organizations in enhancing and sharing knowledge through efforts such as this report will become increasingly important.