Everyone knows that archaeologists study peoples and cultures of the past. But exactly how and what do they study? Archaeology is a science, and as scientists they use evidence to develop conclusions and theories. Evidence for archaeologists comes not just in the form of artifacts (things made or modified by human hands), but also any other direct or indirect trace of what people did in the past. In this sense, archaeology can be defined as the scientific study of the physical traces of past human behavior in order to learn about human culture.

The most likely places to find evidence of past human behavior is in the places where people lived and worked. Such places are called archaeological sites. Since they usually contain large amounts of garbage (i.e. artifacts), archaeological sites are usually identified by the discovery of artifacts either on the exposed surface of the ground or in areas of disturbed earth. Most such sites are found accidentally by farmers and landowners, or by artifact collectors. Archaeologists depend on the help of these people to report the locations of archaeological sites so they can be recorded, studied, and in some cases preserved for future generations.

Once they have chosen a site for study, archaeologists next attempt to determine how large the site is, how deep the deposits extend into the ground, and precisely when portions of the site were occupied by people in the past. If the site is in a freshly-plowed or graded field, artifacts may be scattered on the surface of the ground, immediately revealing the size of the site. This also provides a representative sample of artifacts that can be used to determine the age of the site (artifacts are normally dated by their shape, size, and style of manufacture). On the other hand, if the site is in pasture or woods, archaeologists may also conduct systematic posthole or shovel tests at regular intervals across the site, sifting the dirt through mesh screens to recover any artifacts. The walls, or profiles, of each hole also show layers of earth that show the depth of any plowzone (the upper soil churned by years of repeated agricultural plowing), and may also indicate the presence of midden soil (dark, organically-rich layers of earth created by intensive human occupation). These subsurface tests are normally laid in along a grid established using a compass or surveyor's transit, which is also used to create a detailed topographic map of the site showing its overall size, shape, and elevations.

Following survey and mapping, archaeologists may proceed to more extensive test excavations, designed to gain larger samples of artifacts below the surface, and to expose slightly broader areas of ground in selected locations. Square or rectangular testpits measuring one, two, or more meters on a side are staked in and marked with cord before they are excavated in horizontal layers using flat-shovels, bricklayer's trowels, and other tools. Dirt from each layer is sifted and bagged separately, and records are kept regarding soil color and texture, artifacts found, and any soil discolorations that may indicate the presence of features, representing subsurface pits or postholes excavated and then refilled by the site's previous occupants. Patterns of such features can reveal the presence of ancient houses, walls, activity areas, etc., and can provide many clues to past life at the site.

Since excavation always destroys the original context of all artifacts, soil layers, and features found, archaeologists normally prefer to limit excavations in order to preserve as much as possible of each archaeological site. Nevertheless, in some cases where sites are at risk from destruction, or where important research goals necessitate it, archaeologists expose much larger site areas in order to learn more about a site's history of human occupation.