How and why Europeans evidently ate fish during the medieval millennium as revealed in traditional verbal records, archaeological remains of fish in human waste deposits, and biochemical traces of fish proteins in human skeletal remains. Christian teachings allowed fish during regularly recurring religious taboos on mammal and bird meats. Medical teachings inherited from classical Greco-Roman culture and further elaborated by Muslim and later Christian physicians both advised and constrained fish consumption, as did some folk beliefs. All social ranks recognized fish consumption, especially that of certain large or impressive fishes, as marking high status, wealth, and power. Hence medieval demand for fish was highly stratified. Medievals fished for household subsistence and eventually some people caught fish for sale to other consumers.