In this chapter the role of the ocean on climate and climate change is discussed in terms of the properties of oceans and in terms of the tools available to oceanographers. The details of the Atlantic Meridional Overturning Circulation (AMOC) are described with special reference to motivation, driving mechanisms, heat transport and the ocean's uptake of carbon and the ventilation of the deep ocean. The past changes of the AMOC and the Atlantic climate are also discussed. The chapter ends with a discussion of three questions:. •Why should the AMOC change as a result of climate change?•Can we detect changes in the AMOC?•Is the AMOC already changing as a result of climate change?