A fundamental aim of marking and tracking migratory birds is to link breeding, wintering and stopover areas within discrete populations. Such information allows critical insight into factors influencing populations at several geographical scales and so is fundamental to conservation of migrant birds (Myers et al. 1987; Moore et al. 1995; Sherry and Holmes 1995). For populations with strong connectivity between breeding and wintering sites, population demographics might be modelled using estimates of survivorship, productivity, emigration and immigration (e.g. Burnham 1993; Blums et al. 1996). Populations of birds with loose connectivity between breeding and wintering sites will manifest population levels reflecting factors integrated over broader demographic scales and so complicate our interpretations of population changes. This is also true for those species with poor encounter rates of marked individuals. Despite clear conservation and academic motivations to track migratory birds, useful information is generally lacking for most of the world’s species. In North America, extensive banding programs have resulted in extremely poor recovery rates for most passerines compared to hunted game species (Table 1). It is clear that while programs such as the constant-effort mist-netting stations across Canada and the United States (Dunn et al. 1997; Francis and Hussell 1998) will provide useful information on overall population trends, they will not establish linkages between breeding, stopover and wintering sites for almost all species. New tools are clearly required to address such questions of population linkage (Webster et al. 2001). Fortunately, recent research into the use of intrinsic vs. extrinsic markers shows considerable promise. There are three primary areas of research into the use of intrinsic markers of interest to tracking migratory wildlife. These are the measurement of naturally occurring stable isotopes of several elements and trace element compositions in avian tissues and the use of DNA techniques. This chapter will deal with advances in stable isotope methodologies.