Restriction on importation refers to measures, normally adopted by governments, which restrict the ability of firms to enter foreign markets via imports. The most common restrictions are tariffs, quotas and voluntary export restraints. Tariffs serve to tax imports, thus making them expensive relative to domestic goods. Quotas affect imports directly by restricting the number of units which can come from abroad. Voluntary export restraints, largely confined to the U.S., are similar to quotas in that they restrict quantities. They differ from quotas in that they are not imposed unilaterally by the importing country. Rather, they are agreed to by the exporting country or countries, usually to forestall the imposition of tariffs and/or quotas.