This chapter highlights developments in the past 25 years that have advanced understanding of isotope behavior in food webs, with a view to providing guidance for those interested in forensic back-calculations of diet or habitat of questioned animal tissue. The forensic application of stable isotopes requires an understanding of the ways in which isotopes behave in food webs. All living organisms acquire their tissue isotopic compositions through food webs and it is important to know the basic processes that can alter the isotopic composition of consumers through their diets and so characterize them for forensic investigations. Food webs can be described simply as "diagrams depicting which species in a community interact"; food webs depict both actors and actions, but both the occurrence and magnitude of interactions can change in space and time. The chapter considers how knowledge of food web processes can help eliminate possible origins using isotopes.