text: In situ, controlled perturbation experiments, often conducted over weeks to months, can provide inference concerning the response of natural communities to ocean acidification that is difficult or impossible to derive from laboratory experiments. Studies conducted in situ can include the effects of potentially important factors such as natural variation in planktonic food resources, larval abundance, changes in predators or competitors, as well as oceanographic conditions (e.g. changes in upwelling intensity). Drawing on the experience of Free Air CO Enrichment (FACE) experiments used to investigate the response of terrestrial plant communities to rising atmospheric CO levels, the scientific community has developed an analogous approach, Free Ocean CO Enrichment (FOCE) experiments, for studying marine communities, and to complement a range of experimental methods and technologies for ocean acidification studies research. FOCE was first proposed and implemented by researchers at the Monterey Bay Aquarium Research Institute (MBARI).