Carbon dioxide removal has emerged as a critical component of efforts to achieve the temperature targets of the Paris Agreement, and thus to avoid catastrophic climate change. Most experts believe that effectuating the massive scale of carbon removal contemplated in recent models will require a portfolio of both terrestrial and marine carbon removal options. One of the most often discussed, and researched, marine-based approaches is ocean iron fertilization (OIF). OIF is an approach that would seek to seed portions of the oceans with iron to stimulate phytoplankton production, which could ultimately result in uptake of more carbon dioxide by these species and subsequent burial of carbon dioxide in ocean sediments and drawdown of more carbon dioxide from the atmosphere. This chapter seeks to explain the science behind OIF, the potential effectiveness of the approach, potential risks of large-scale deployment, and critical questions of monitoring and verifying sequestration of carbon through deployment.