Seabirds have a global distribution, are numerous throughout the world’s oceans, and have been used for decades to track and understand changes in the marine environment. They are dependent on a variety of ecosystems, including terrestrial, coastal, and pelagic, and are thus vulnerable to both marine and terrestrial environmental stressors. This chapter examines impacts on seabird populations that occur in the marine environment and are global in nature (touch more than one ocean basin). Both widespread (i.e., climate change induced alterations to marine food webs and sea level rise) and more point-source impacts (i.e., incidental bycatch in fisheries, hunting) are discussed. Additionally, natural occurrences in marine ecosystems (i.e., oceanographic regime shifts, parasites) and issues related to anthropogenic activities (i.e., plastic and oil pollution) are covered. Lastly, we discuss marine protected areasand other efforts aimed at conserving global seabird populations, including colony restoration, community-based research, and international conservation actions.