Metrication (or metrification) refers to the introduction of the SI metric system as the international standard for physical measurements—a long-term series of independent and systematic conversions from the various separate local systems of weights and measures. Metrication began in France in the 1790s and spread widely during the following two centuries.

Based on an old U.S. Metric Association survey it is commonly cited that only the United States of America, Liberia and Myanmar (Burma) have not officially adopted the metric system. But Guyana, for example, has officially adopted the metric system every 3-5 years, on average, since 1981, but each time with little success. Antigua, again 'officially' metric, is only slowly moving toward implementing the metric system.

The United Kingdom and Saint Lucia are officially in the process of conversion, although the UK has been granted permanent exemptions by the EU for the mile and yard in road markings, and for the pint for beer and milk. In 2007, the European Commission also announced that it was to abandon the requirement for metric-only labelling on packaged goods, and to allow dual metric-imperial marking to continue indefinitely. Other countries in the former British Empire completed metrication during the second half of the 20th century, the most recent being the Republic of Ireland, which finalised conversion in early 2005.

The United States and the United Kingdom see active opposition to metrication today, the main objections being based in localism, tradition, cultural aesthetics, economic impact, repeating decimal notation when dividing by some numbers (3,6,7,9 etc), or distaste for measures viewed as "foreign". While other countries, like France and Japan, also had significant popular opposition at one time for similar reasons, metrication is now accepted.