The Trump administration claims the US relies too much on other countries for its metals, and that it couldn't make enough weapons or vehicles using its own industry if a war broke out.
Critics point out the US gets most of its steel from Canada and the EU - staunch US allies.
In theory, taxing foreign steel and aluminium will mean US companies will buy local steel instead.
The thinking is that will boost the US steel and aluminium industries, as more companies will want to buy their goods.
Steel and aluminium prices will go up in the US because there will be less of these goods coming in from abroad - so the greater demand for local steel will push up the price, lifting profits for steel makers.