RATIONALE: Carbon (δ 13C) and nitrogen (δ 15N) analysis has been extensively used to investigate the importance of marine foods in the diet of archaeological populations in the North Atlantic Islands; however, few faunal studies exist to aid the interpretation of results. Palaeoenvironmental modelling of δ 13C and δ 15N values is crucial in determining whether changes in the stable isotope values are a result of dietary change, rather than temporal or geographical fluctuations in carbon and nitrogen. Investigating faunal dietary behaviour can provide an insight into past foddering and land management strategies. METHODS: Detailed sampling of wild and domestic species for bulk collagen analysis was undertaken in order to characterise geographical variations in δ 13C and δ 15N values in the Outer Hebrides and Orkney. Samples from the Neolithic to the Norse period were analysed to assess temporal and geographical variations in δ 13C and δ 15N values, in addition to determining the contribution of marine foods to the diet of local fauna. RESULTS: A δ 15N shift of 1‰ was observed between the Outer Hebrides and Orkney in the Neolithic and Iron Age. A geographical variation in δ 13C values was observed in the Norse period between Orkney and the Outer Hebrides. Temporal fluctuations in δ 13C and δ 15N values demonstrate variations in foddering practices of sheep in the Outer Hebrides. Pig specimens from the Outer Hebrides demonstrated evidence of marine food consumption in the Iron Age. CONCLUSIONS: Faunal dietary behaviour can act as a vital indicator of the importance of marine resources in the past. Characterisation of faunal δ 13C and δ 15N values geographically and temporally is crucial in our interpretation of human dietary behaviour.