For the past 15 years, a succession of stable isotope studies have documented the abrupt dietary transition from the Mesolithic to the Neolithic in Western and Northern Europe. Portugal, with its Late Mesolithic shell middens and burials apparently coexisting with the earliest Neolithic, further illustrates the nature of that transition. Individuals from Neolithic contexts there had significantly different diets to their Mesolithic counterparts. No evidence was found for a transitional phase between the marine-oriented Mesolithic subsistence regimes and the domesticated, terrestrial Neolithic diet. Two later Neolithic individuals, however, showed evidence for partial reliance on marine or aquatic foods. This raises questions about the possible persistence of marine dietary regimes beyond the Mesolithic period. This article is followed by a brief note by Mary Jackes and David Lubell.