Open system modification of magmas during their storage and transport through the mantle and crust is now widely recognized (or suspected) as masking many primary features of these liquids. Nevertheless, magmas derived from the mantle and the crust and from specific tectonic settings possess distinctive trace element signatures with presumably are imposed by source compositions or specific petrogenetic processes that also control isotopic compositions. We briefly summarize evidence to indicate the scale and importance of regional isotopic variations in the mantle and then discuss subsequent open system modifications of magmas during their ascent. Specific case studies of continental flood basalts (Deccan, Snake River Plain), volcanic arc magmas (Andes), and other intraplate magmatic suites (Kane Springs Wash and Yellowstone eruptive centers) are discussed to illustrate different styles of trace element/isotope variations. These examples demonstrate the sorts of constraints provided by combined trace element and isotopic studies concerning mixing and assimilation processes that lead to modification of evolving magmas, as opposed to compositional variations inherited from magma soure regions. These examples further illustrate effects of source regions. These examples further illustrate effects of source mixing and magma-crust or magma-fluid interactions. Despite uncertainties in compositions of assimilant or mixing end-members, trace element/isotopic compositional vectors can provide useful constraints on the nature of such end-members and on the dominant petrogenetic process involved. Obviously, this approach is most successful when applied to discrete eruptive centers where the magmas are most likely to be cogenetic.