Encounters between black-billed magpies (Pica pica) and a variety of natural predators were observed during 3 breeding seasons in Wind Cave National Park, South Dakota. How a magpie responded to a potential predator appeared to depend on (1) the type of predator and the threat it posed to magpie eggs, nestlings, fledglings, and adults; (2) the behavior of the predator; and (3) the reproductive stage of the magpie. Raptors were the most frequently encountered potential predators, with magpies reacting more strongly to falcons than to hawks. Reactions to crows and squirrels were most frequent and intense during laying and incubation, while raptors in flight and coyotes were responded to most vigorously during the second half of the nestling period and the first two weeks of fledgling. Perched raptors were almost always mobbed vigorously. Diving to within 2 m of a predator appeared to be effective in driving it away. The roles of chasing and alarm calling were less clear, but in addition to alerting mates and offspring to danger, such behavior would impede efficient hunting by the predator and so might contribute to its departure. Only one successful act of predation was observed, but the evidence suggested that owls, hawks and falcons were responsible for most fledgling and adult mortality. No clear cases of nest predation by crows or squirrels occurred, but some clutches of eggs and broods of young nestlings disappeared. Although mates usually mobbed predators together, males were slightly more active, possibly because males were larger and because females were often occupied in the nest with incubation and brooding. Observations of wild fledglings and hand-raised magpies suggest that magpies have an innate fear response to a variety of stimuli and that fledglings gradually learn what to continue fearing from parents, other magpies, and their own experiences. I suggest that although parental investment theory may be basically correct in predicting that young increase in value to their parent as they approach independence, a variety of other factors may affect anti-predator behavior, such as (1) the types of predators likely to be encountered and the relative danger they pose to different age classes; (2) the ability of the parents to successfully drive off a particular predator; (3) the ability of the parents to re-nest that year; (4) the likelihood that a pair will be able to fledge young the following year.