The BIPM's original mission was to promote and extend the metric system throughout the world by: maintaining the International Prototypes of the metre and the kilogram, constructing copies of the prototypes, maintaining the new prototypes, comparing the various national systems of measurement standards with the prototypes of the kilogram and the metre, and perfecting the measurement methods in order to promote metrology, thereby ensuring worldwide conformity of measurements. To accompany industrial development, the scope of the BIPM was extended to new areas of technology – electrical measurement standards (1937), photometric measurement standards (1937), ionizing radiation measurement standards (1960), measurement standards in time (1988), and measurement standards in chemistry (2000).

The BIPM was charged with providing the basis for a single, coherent system of physical measurements throughout the world. In 1875, the chosen system of units was the metric system. Over time this decision evolved, and in 1960 it was decided at the 11th General Conference on Weights and Measures (CGPM) that the chosen system of units was to be the International System of Units (SI). The SI is not static but evolves to match the world's increasingly demanding requirements for measurements at all levels of precision and in all areas of science and technology.

The definitions of the SI base units have changed over the years to meet the changing requirements and an increasing need for accurate industrial and commercial measurements. Within the SI, all units of measurement can be reduced to a small number of fundamental base units. The methods of measurement and the standards themselves undergo constant progress and renewal, and the finer or more accurate the measurements – the greater the precision in the definition of the units of measurement. Indeed, the work on fundamental standards or constants, and their extension into new domains of application, carried out by national metrology laboratories and the BIPM will probably never end.

This evolution is reflected in the changing definition of the metre away from a material object or artefact. Although universal, the implementation of the metre as a unit of length, defined as a proportion of the quarter meridian, was difficult. The original 1799 metre was based on measurements made along a meridian from Dunkerque to Barcelona and represented an attempt to realize this length. Such a physical artefact remained in the definition of the metre until 1960 when the definition was replaced by one based on a measurement of the wavelength of orange light emitted by excited atoms of an isotope of krypton. This definition was eventually superseded in 1983 by the present one, which is derived by assigning a defined value to the speed of light in free space. Today, the SI unit of the metre is "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second", thus fixing the velocity of light at 299 792 458 m s^–1.

	The difficulty lies, not in new ideas, but in escaping the old ones, which ramify into every corner of our minds. John Maynard Keynes

The fields of application of metrology are unlimited, since new areas of study become accessible to human knowledge, each requiring measurement instruments and sometimes the creation of new units of measure. Metrology is therefore a branch of science which is continually challenging ideas of what is possible or what may be possible.