Sea turtles spend the majority of their lives in coastal or pelagic waters, making in-water sources of mortality critical to population viability. Sea turtles have been negatively impacted by a number of human-mediated factors including oil spills (Antonio et al., 2011), contaminants (van de Merwe et al., 2010; Swarthout et al., 2010; Komoroske et al., 2011; Stewart et al., 2011), and other types of marine pollution, namely debris ingestion and entanglement (Lazar and Gracan, 2011; do Sul et al., 2011). Coastal and in-water shoreline development also have been shown to degrade ocean habitat, which can negatively affect resident turtles (Harewood and Horrocks, 2008; Pike, 2008). While all of these factors likely have some negative effect on sea turtle populations, the human activity that has the largest impact on sea turtles is fisheries bycatch (Lewison et al., 2004a; Wallace et al., 2011). Although directed take of turtles is one form of fisheries impact, and in some regions opportunistic take of captured turtles is still prevalent (Alfaro-Shigueto et al., 2011), turtles are generally an unwanted and unwelcome byproduct of fishing activities. Because fishing is an important source of protein and livelihood for millions of people worldwide, incidental capture, or bycatch, of sea turtles continues to be the most pressing human impact on sea turtle populations globally. In this chapter, we review the current state of knowledge about global marine turtle bycatch, including how characteristics of sea turtle biology and fishing practices interact to result in bycatch, assessments of population-level impacts of turtle bycatch, descriptions of where and how turtle bycatch occurs across distinct fisheries sectors, a summary of techniques and approaches to bycatch reduction, and new ways forward for bycatch research and management.