In archaeology, a biface is a two-sided stone tool and is used as a multi purposes knife, manufactured through a process of lithic reduction, that displays flake scars on both sides. A profile view of the final product tends to exhibit a lenticular shape (i.e., as a convex lens). Bifacial artifacts can be made on large flakes or blocks, and may be grouped into numerous distinct classes.

For the purposes of this article, four classes are defined :

While Class IV bifaces are referred to as "formalized tools", bifaces from any stage of a lithic reduction sequence may be used as tools. (Also, other biface typologies make five divisions rather than four).

Examples of bifaces first appeared 1.6mya in the later Oldowan (Mode I), called the "developed Oldowan" by Mary Leakey, but became more abundant in mode II Acheulean industries that appear in what is now Southern Ethiopia around 1.4 million years ago, although some of the best examples come from 1.2 million year old deposits in Olduvai Gorge. They are also known in Mousterian industries. In North America, bifaces make up one of the dominant tool industries, starting from the terminal Pleistocene and continuing throughout the Holocene. For example, the Folsom point and Clovis point traditions (collectively known as the fluted points) are associated with Paleo Indians, some of the first people to colonize the new world (see Models of migration to the New World). Further, biface technology is almost unknown in Australian prehistory.