The polar oceans interact with both sea ice, formed in situ at the ocean surface, and land ice, flowing under gravity from the land onto the ocean surface. This ice–ocean interaction has profound consequences for the ocean and climate in a number of ways: a change in ocean surface albedo and surface energy balance where there is ice cover compared with open ocean, a change in global sea level when land ice flows into the ocean, and a transformation of water masses through melting or freezing of ice which subsequently influences the global conveyor belt. Another type of ice–ocean interaction, less well understood, is that between marine permafrost at the seafloor and the overlying ocean waters. Collectively, we refer to sea ice, land ice, and marine permafrost as the marine cryosphere. In this chapter, we review current understanding of the interaction of the marine cryosphere with the global ocean and discuss emerging technologies to improve observations and numerical modeling of these interactions. Projections for the state of the marine cryosphere into the current century and beyond are reviewed.