We open this Special Feature on stable isotopes in mammalian research with a beginner's guide, an introduction to the novice and a refresher to the well-versed. In this guide we provide the background needed to understand the more advanced papers that follow. We describe the basic principles of isotopic fractionation and discrimination, briefly explain the processes that govern isotopic incorporation into animal tissues, list some innovative studies, and provide cautionary notes and caveats. In addition to discussing the uses of natural abundance we present the concepts and applications of enriched isotopes and the potential combination of these 2 methodologies. We end with descriptions of analytical and conceptual developments that we believe will be cardinal to the future of isotopic analyses in mammalian research.