Biostratigraphy is the use of fossils to date rocks. It has allowed the creation of the New Zealand Geological Time Scale.

It is based on the the changing sequence of life on Earth, with different species evolving, dispersing and becoming extinct through time.

Sedimentary rocks that have been deformed, tilted, or even overturned can be correctly understood through knowledge of biostratigraphy, even though they may now be upside down (see Superposition).

Hard-shelled marine fossils are the most useful for biostratigraphy, particularly those that are abundant and widespread. Other fossil species may be too rare or poorly preserved to be useful.

Planktic species (that float and live in the surface water of the oceans) are best because they are widely distributed and independent of the type of seafloor sediment. In contrast, benthic (seafloor) species may be restricted to particular sediments.

Rapidly evolving and wide-ranging successions of species are ideal because they allow the recognition of narrow time intervals in different places. Many different fossil species are used as New Zealand indicators of biostratigraphy

As well as being important for the study of evolution, plate tectonics, climate change and sea level change, biostratigraphy is also used in the global search for oil and gas.