text: As studies of the consequences of ocean acidification for marine organisms and ecosystems expanded rapidly over the past decade, the methods employed to evaluate the effects of expected future changes in ocean chemistry have become more sophisticated. Initial studies frequently involved measurements of the survival or physiological response of individuals of marine species to large changes in pCO2 or pH, while held in small containers under laboratory conditions. This approach increased the level of understanding of the effects of these environmental changes on individual species but provided little information concerning the response of natural assemblages of interacting species, in which the direct impacts of ocean acidification as well as their cascading indirect consequences (e.g. changes in the intensity of interaction strengths among predators or competitors) may be evident. Pelagic mesocosm experiments that examined the response of natural plankton communities to controlled pH perturbations helped move methods of ocean acidification research toward more comprehensive studies of whole communities and embedded processes under mostly natural conditions. The FOCE approach represents an analogous advance for benthic assemblages, by allowing examination of the direct effects of acidification on particular species, but also potential changes in interactions among species. Moreover, FOCE methods provide precise control of pH, while allowing many other parameters to vary naturally. Like mesocosm studies, FOCE methods exploit the advantages of studying a natural community under mostly natural ranges of environmental variability.