Before Present (BP) years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1 January 1950 AD as the origin of the age scale, reflecting the fact that radiocarbon dating became practicable in the 1950s. The abbreviation "BP" has also been interpreted to mean Before Physics.

The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy.

This usage differs with the recommendation by van der Plicht & Hogg, followed by the Quaternary Science Reviews, both of which requested that publications should use the unit "a" [for year] and reserve the term "BP" for radiocarbon estimations.

Beginning in 1954, metrologists established 1950 as the origin year for the BP scale for use with radiocarbon dating, using a 1950-based reference sample of oxalic acid:

The problem was tackled by the international radiocarbon community in the late 1950s, in cooperation with the U.S. National Bureau of Standards. A large quantity of contemporary oxalic acid dihydrate was prepared as NBS Standard Reference Material (SRM) 4990B. Its ¹⁴C concentration was about 5% above what was believed to be the natural level, so the standard for radiocarbon dating was defined as 0.95 times the ¹⁴C concentration of this material, adjusted to a ¹³C reference value of –19 per mil (PDB). This value is defined as "modern carbon" referenced to AD 1950. Radiocarbon measurements are compared to this modern carbon value, and expressed as "fraction of modern" (fM). "Radiocarbon ages" are calculated from fM using the exponential decay relation and the "Libby half-life" 5568 a. The ages are expressed in years before present (BP) where "present" is defined as AD 1950.