The benthic environment refers to the region defined by the interface between a body of water and the bottom substrate, including the upper part of the sediments, regardless of the depth and geographical location. Hence, benthic environments, their organisms, and their ecosystems are highly variable as they encompass the full depth range of the oceans with associated changes in physical and chemical properties as well as differences linked to latitudinal and geographical variation. The effects of ocean acidification on the full range of different benthic organisms and ecosystems are poorly known and difficult to ascertain. Nevertheless, by integrating our current knowledge on the effects of ocean acidification on major benthic biogeochemical processes, individual benthic organisms, and observed characteristics of benthic environments as a function of seawater carbonate chemistry, it is possible to draw conclusions regarding the response of benthic organisms and ecosystems to a world of increasingly higher atmospheric CO2 levels. The fact that there are large-scale geographical and spatial differences in seawater carbonate system chemistry (see Chapter 3), owing to both natural and anthropogenic processes, provides a powerful means to evaluate the effect of ocean acidification on marine benthic systems. In addition, there are local and regional environments that experience high-CO2 and low-pH conditions owing to special circumstances such as, for example, volcanic vents (Hall- Spencer et al . 2008 ; Martin et al . 2008 ; Rodolfo-Metalpa et al . 2010), seasonal stratification (Andersson et al . 2007), and upwelling (Feely et al . 2008 ; Manzello et al . 2008 ) that may provide important clues to the impacts of ocean acidification on benthic processes, organisms, and ecosystems. The objective of this chapter is to provide an overview of the potential consequences of ocean acidification on marine benthic organisms, communities, and ecosystems, and the major biogeochemical processes governing the cycling of carbon in the marine benthic environment, including primary production, respiration, calcification, and CaCO3 dissolution. The depth of the euphotic zone, i.e. the depth of water exposed to sufficient sunlight to support photosynthesis, varies depending on a range of factors affecting the clarity of seawater, including river input and run-off to the coastal ocean, upwelling, mixing, and planktonic production.