This chapter provides a critical review of international research on biological weed control as it applies to weeds of temperate grasslands. The chapter outlines the ways in which ‘classical’ biocontrol agents, due to their natural ability to spread and find their host plant, are appropriate for the grassland on a steep terrain not readily accessible with farm machinery. ‘Bioherbicides’, by contrast, require the use of tractors, sprayers or spreaders and are therefore restricted to land accessible with farm machinery. Based on the published literature, including many historical reviews and the authors’ own research experience, the chapter identifies the constraints to, and opportunities for, the effective biological control of weeds in the world’s temperate grasslands.