Stratigraphy is the study of the successive geological or archaeological layers of a given site. Such layers, called stratigraphic units (SU), are the result of accumulating sediment, human material remains, animal or plant remains and, more generally, the traces of past events occurring on a given site.

The study of stratigraphic units enables archaeologists to characterize and sometimes date a succession of events that may or may not have been caused by human activities. The order and position of the layers that comprise a given stratigraphic sequence provide a relative chronology of events, that is to say, they can be dated in relation to each other. By comparing this relative chronology to various datable elements contained in certain layers, such as pottery, coins or other items that can provide a date (also called index fossils or chronological markers), archaeologists can establish points that anchor it to an absolute chronology. 
Stratigraphy, a fundamental principle of contemporary archaeology, requires methodical excavations and a rigorous and systematic measurement protocol. Stratigraphic units or layers can be distinguished from each other by their physical characteristics (consistency, colour, composition, hade/slope, etc.) and the archaeological material they contain. Each SU is documented by saving a detailed file, by making drawings in plan and cross-section, and by taking photographs. All these documents enable the archaeologist to undertake a stratigraphic study, even after certain layers have been removed for the purpose of attaining underlying layers. 
In underwater archaeology, stratigraphic studies are less common than in excavations on land because wrecks are hermetic sites originating from a brutal yet isolated event. However other submerged sites, such as harbours, shore constructions, middens, bridges, fish keeps and stilt dwellings, contain information that has been deposited over a period of time and thus feature stratigraphic sequences which are similar to those observed on dry land.