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The analysis of natural stable isotope ratios has created a methodological upheaval in animal ecology. Because the distribution of stable isotopes in organisms follows reliable patterns, their analyses have become established useful methods for animal ecologists. However, because animal ecologists have adopted a phenomenological approach to the use of stable isotopes, the mechanisms that create isotope variation patterns remain unexplored. In this chapter we develop mass balance models that can provide a mechanistic, and hence predictive, foundation for animal isotopic ecology. We review and elaborate the current mixing models used to reconstruct animal diets and develop new mathematical models to explain one of the most widely used patterns in animal isotopic ecology: the enrichment in 15N observed across trophic levels. The construction of element and isotope budgets is central to testing the mass balance models described in this chapter. Because the concept of a budget is central to all of animal physiological ecology, the development of a mechanistic and predictive framework for isotopic animal ecology falls naturally on physiological ecologists. We argue that progress in isotopic animal ecology hinges on laboratory experiments that explore mechanism, the documentation of pattern in the field, and the integration of mechanism and pattern by theory.