Archaeology is a historical social science concerned with study of past societies and cultures through material traces, called the archaeological record. These traces may have been left by early human ancestors, millions of years ago—or by contemporary people as recently as yesterday. Study may be text aided among literate societies; most of the human past, however, involved societies with no writing, what some call “prehistory.” Research involves examination of artifacts (objects of human manufacture), features (arrangements of artifacts, construction elements, or other items), ecofacts (naturally occurring items that inform about human lives, such as soils), and sites (locations in which one or more of the foregoing occur). Archaeological ethics promotes growing collaboration with descendant communities in framing research goals and techniques. For that reason, and because the research process commonly destroys the archaeological record, practitioners increasingly seek less invasive or destructive methods. In all cases, archaeologists employ systematic scientific methods for recovery and study of material remains, documenting as fully as possible the materials encountered along with the temporal (stratigraphic) and spatial (association) contexts in which they were found. Archaeology is inherently interdisciplinary, calling on expertise in such fields as geology, biology, ethnology, and history. Interpretive aims vary with the research project, and with the theoretical orientation of its directors. In that way, theory is central to archaeology. Although in many parts of the world, archaeology is a discipline unto itself, in the United States it is most commonly considered part of anthropology. Exceptions are classical archaeology, allied more closely with history and art history, and historical archaeology, often teamed in the United States with history and American studies.