The US has introduced a 25% tariff on all steel and aluminum imports from around the world.
In response, the EU has said it will impose tariffs on US goods worth billions of dollars, stoking fears of a global trade war.
President Donald Trump has already imposed 25% tariffs on other imports from Mexico and Canada - with some exemptions - and a 20% levy on Chinese goods.
Tariffs are taxes charged on goods imported from other countries.
The companies that bring the foreign goods into the country pay the tax to the government.
Typically, tariffs are a percentage of a product's value. A 20% tariff on Chinese goods means a product worth $10 (£7.76) has an additional $2 charge.
Firms may choose to pass on some or all of the cost of tariffs to customers.
The US has typically charged lower tariffs on goods than other countries.
Trump's new tariffs - and his promise to introduce further "reciprocal" levies on goods from other countries - could lead to a sharp increase in the prices people pay for goods in the US and around the globe.
