Metrologists working in different areas specialize in different types of measurements. At the highest scientific level, metrologists ensure the consistency of the International System of Units (SI), which built on the early units of the Metric System and which was formally created in 1960. Their work usually involves research into the definitions of the units and ways of realizing them with sufficient accuracy to meet the needs of society and the world of scientific research. Legal metrologists are involved in aspects of metrology in the regulated sector, which directly concern consumers. Both metrologies are essential in ensuring consistent national measurement systems, traceable to international standards; thereby establishing that measurements and tests made in different countries can be regarded as equivalent.

The maintenance of the world's system of units takes many forms, from direct dissemination of units (as in the case of mass and time) to coordination through international comparisons of national measurement standards (as in length, electricity and ionizing radiation). Such comparisons are coordinated by the International Committee for Weights and Measures, the CIPM.

The creation in 1999 of the CIPM Mutual Recognition Arrangement (MRA) marked a major advance in the internationalization of metrology. A means of increasing confidence in the technical abilities of participants from laboratories around the world to make equivalent measurements and enable the provision of calibration certificates that are validated, verified and accepted by all signatories represents a significant contribution to the reduction to technical barriers to trade.

Some people claim that the CIPM MRA is as influential as the Metre Convention itself. Time will tell, but it is clear that metrologists are actively involved in practical research to produce results which bring potentially huge benefits to society at large. One recent estimate is that the impact of the CIPM MRA in reducing technical barriers to trade is worth over $4 billion.