Fossils are the mineralized or otherwise preserved remains -- or other traces (such as footprints) -- of animals, plants, and other organisms. The totality of fossils and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record. The study of fossils is called paleontology.

The word fossil is derived from the Latin word fossus, which means "having been dug up".

Fossilization is actually a rare occurrence because natural materials tend to decompose. In order for an organism to be fossilized, the remains normally need to be covered by sediment as soon as possible. However there are exceptions to this, such as if an organism becomes petrified or comes to rest in an anoxic environment such as at the bottom of a lake. There are several different types of fossils and fossilization processes.

Fossils usually consist of traces of the remains of the organism itself. However, fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or feces of a dinosaur or reptile. These types of fossil are called trace fossils, as opposed to body fossils. Finally, past life leaves some evidences that cannot be seen but can be detected in the form of chemical signals; these are known as chemical fossils (for lack of a better term).

The oldest known structured fossils are most likely stromatolites. Now understood to be formed by the entrapment of minerals by mucous-like sheets of cyanobacteria, the oldest of these formations dates from 3.5 billion years ago. Even older deposits (3.8 billion years old) of heavy carbon that are indicative of even earlier life are currently proposed as the remains of the earliest known life on Earth.