Metrology is defined by the International Bureau of Weights and Measures (BIPM) as "the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology." 
Metrology is a very broad field and may be divided into three subfields:
- Scientific or fundamental metrology concerns the establishment of measurement units, unit systems, the development of new measurement methods, realisation of measurement standards and the transfer of traceability from these standards to users in society.
- Applied or industrial metrology concerns the application of measurement science to manufacturing and other processes and their use in society, ensuring the suitability of measurement instruments, their calibration and quality control of measurements.
- Legal metrology concerns regulatory requirements of measurements and measuring instruments for the protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade.
A core concept in metrology is (metrological) traceability, defined as "the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties." The level of traceability establishes the level of comparability of the measurement: whether the result of a measurement can be compared to the previous one, a measurement result a year ago, or to the result of a measurement performed anywhere else in the world.
Traceability is most often obtained by calibration, establishing the relation between the indication of a measuring instrument and the value of a measurement standard. These standards are usually coordinated by national laboratories: National Institute of Standards and Technology (USA), National Physical Laboratory, UK, etc.
Metrology has existed in some form or another since antiquity. The earliest forms of metrology were simply arbitrary standards set up by regional or local authorities, often based on practical measures such as the length of an arm. The earliest examples of these standardized measures are length, time, and weight. These standards were established in order to facilitate commerce and record human activity.
Little progress was made with regard to proto-metrology until various scientists, chemists, and physicists started making headway during the Scientific Revolution. With the advances in the sciences, the comparison of experiment to theory required a rational system of units, and something more closely resembling modern metrology began to come into being. The discovery of atoms, electricity, thermodynamics, and other fundamental scientific principles could be applied to standards of measurement, and many inventions made it easier to quantitatively or qualitatively assess physical properties, using the defined units of measurement established by science.
Modern metrology has its roots in the French Revolution, with the political motivation to harmonize units all over France and the concept of establishing units of measurement based on constants of nature, and thus making measurement units available "for all people, for all time". In this case deriving a unit of length from the dimensions of the Earth, and a unit of mass from a cube of water. The result was platinum standards for the meter and the kilogram established as the basis of the metric system on June 22, 1799. This further led to the creation of the Système International d'Unités, or the International System of Units. This system has gained unprecedented worldwide acceptance as definitions and standards of modern measurement units. Though not the official system of units of all nations, the definitions and specifications of SI are globally accepted and recognized. The SI is maintained under the auspices of the Metre Convention and its institutions, the General Conference on Weights and Measures, or CGPM, its executive branch the International Committee for Weights and Measures, or CIPM, and its technical institution the International Bureau of Weights and Measures, or BIPM.
As the authorities on SI, these organizations establish and promulgate the SI, with the ambition to be able to service all. This includes introducing new units, such as the relatively new unit, the mole, to encompass metrology in chemistry. These units are then established and maintained through various agencies in each country, and establish a hierarchy of measurement standards that can be traced back to the established standard unit, a concept known as metrological traceability. The U.S. agencies holding this responsibility is known as the National Institute of Standards and Technology, or NIST; and the American National Standards Institute (ANSI).
Industry-specific metrology standards
In addition to standards created by national and international standards organizations, many large and small industrial companies also define metrology standards and procedures to meet their particular needs for technically and economically competitive manufacturing. These standards and procedures, while drawing in part upon the national and international standards, also address the issues of what specific instrument technology will be used to measure each quantity, how often each quantity will be measured, and which definition of each quantity will be used as the basis for accomplishing the process control that their manufacturing and product specifications require. Industrial metrology standards include dynamic control plans, also known as “dimensional control plans”, or “DCPs”, for their products.
In industrial metrology, several issues beyond accuracy constrain the usability of metrology methods. These include 1. The speed with which measurements can be accomplished on parts or surfaces in the process of manufacturing, which must match the TAKT Time of the production line. 2. The completeness with which the manufactured part can be measured such as described in High-definition metrology, 3. The ability of the measurement mechanism to operate reliably in a manufacturing plant environment considering temperature, vibration, dust, and a host of other potential hostile factors, 4. The ability of the measurement results, as they are presented, to be assimilated by the manufacturing operators or automation in time to effectively control the manufacturing process variables, and 5. The total financial cost of measuring each part.
At the base of metrology is the definition, realisation and dissemination of units of measurement. Physical or chemical properties are quantised by assigning a property value in some multiple of a measurement unit.
The basic 'lineage' of measurement standards are:
- The definition of a unit, based on some physical constant, such as absolute zero, the freezing point of water, etc.; or an agreed-upon arbitrary standard.
- The realisation of the unit by experimental methods and the scaling into multiples and submultiples, by establishment of primary standards. In some cases an approximation is used, when the realisation of the units is less precise than other methods of generating a scale of the quantity in question. This is presently the situation for the electrical units in the SI, where voltage and resistance are defined in terms of the ampere, but are used in practice from realisations based on the Josephson effect and the quantised Hall effect.
- the transfer of traceability from the primary standards to secondary and working standards. This is achieved by calibration.
Theoretically, metrology, as the science of measurement, attempts to validate the data obtained from test equipment. Though metrology is the science of measurement, in practical applications, it is the enforcement, verification and validation of predefined standards for precision, accuracy, traceability, and reliability.
- Accuracy is the degree of exactness which the final product corresponds to the measurement standard.
- Preciseness refers to the degree of exactness which a measuring instrument can determine accuracy (actually, inaccuracy).
- Reliability refers to the consistency of accurate results over consecutive measurements.
- Traceability refers to the ongoing validations that the measurement of the final product conforms to the original standard of measurement.
(Fundamentals of Dimensional Metrology, Ted Busch, Wilkie Bros Foundation, Delmar Publishers, ISBN 0-8273-2127-9)
These standards can vary widely, but are often mandated by governments, agencies, and treaties such as the International Organization for Standardization, the Metre Convention, or the FDA. These agencies promulgate policies and regulations that standardize industries, countries, and streamline international trade, products, and measurements. Metrology is, at its core, an analysis of the uncertainty of individual measurements, and attempts to validate each measurement made with a given instrument, and the data obtained from it. The dissemination of traceability to consumers in society is often performed by dedicated calibration laboratory with a recognized quality system in compliance with such standards. National laboratory accreditation schemes have been established to offer third-party assessment of such quality systems. A central requirement of these accreditations is documented traceability to national or international standards.
Some common standards include:
- ISO 17025:2005 - General Requirements for Calibration Laboratories
- ISO 9000 - Quality Systems Management
- ISO 14000 - Environmental Management
- 21 CFR Part 210/211 - FDA Regulations concerning GMP (Good Maintenance Practices) Quality Systems
- 21 CFR Part 110 - FDA Regulations concerning Food Industry GMP's
- Accuracy and precision
- Test method
- Data analysis
- Dimensional metrology
- Certified reference materials
- Organisation Internationale de Metrologie Legale. (2000), International Vocabulary of Terms in Legal Metrology, [Online] http://www.oiml.org/publications/V/V001-ef00.pdf. (Latest version draft can be downloaded at http://www.ncsli.org/vim/wg2_doc_N318_VIM_3rd_edition_2006-08-01%20(3).pdf)
- Bureau International des Poids et Mesures. (2005), "What is metrology", Copyright BIPM 2004, [Online] http://www.bipm.org/en/bipm/metrology/.
- Sarle, W. (1995), Measurement theory: Frequently asked questions, Copyright 1995 by Warren S. Sarle, Cary, NC, USA [Online] SAS Institute web pages: ftp://ftp.sas.com/pub/neural/measurement.faq
- Bureau International des Poids et Mesures. (2000), The International System of Units (SI), [Online] BIPM web pages: http://www.bipm.fr/enus/3_SI/
- Bureau International des Poids et Mesures. (2000), The Convention of the meter, [Online] BIPM web pages: http://www.bipm.fr/enus/1_Convention/
- Melville, D.J. (2001). Sumerian metrological numeration systems, Mesopotamian Mathematics, [Online] St. Lawrence University web pages, http://it.stlawu.edu/%7Edmelvill/mesomath/sumerian.html
- National Institute of Standards and Technology. (1999), The NIST Reference of Constants, Units, and Uncertainty, [Online] NIST web pages: http://physics.nist.gov/cuu/index.html
- National Institute of Standards and Technology / Sematech. (n.d.). Engineering Statistics Handbook. [Online] NIST web pages: http://www.nist.gov/itl/div898/handbook/
- National Physical Laboratory, UK - National Measurement Laboratory - Metrology related resources including many free PDF downloads including Good Practice Guides: [Online] http://www.npl.co.uk/
- Ken Alder, "The Measure of All Things", Little, Brown 2002. (An historical account on the origin of the metric system, the meridian project).
- Kimothi, S. K., "The Uncertainty of Measurements: Physical and Chemical Metrology: Impact and Analysis", 2002, ISBN 0873895355
- Measurement Uncertainties in Science and Technology, Springer 2005
- Proposal for a New Error Calculus
- Estimation of Measurement Uncertainties — an Alternative to the ISO Guide
- Bureau International des Poids et Mesures (BIPM)
- National Institute of Standards and Technology (NIST)
- National Physical Laboratory
- U.S. Naval Observatory
- National Conference of Standards Laboratories (NCSL)
- Calibration in the pharmaceutical, biotechnology, and medical device industries.
- International Standardization Organization
- Agilent Metrology Forum
- Presentation about Product Quality planning that includes a typical industry “Dimensional Control Plan”
- National Metrology Network of Chile
- National Metrology Laboratory of Malaysia
- UNC Charlotte Center for Precision Metrology