Offering a law and economics perspective, this chapter uses the database of US 'international agreements' (defined in the US Case Act as legally binding agreements other than 'treaties' in the sense of Article II:2 of the US Constitution) to analyse to what extent we can see an increase of informality away from formal treaties (in the sense of the US Constitution) to international agreements (who remain binding under international law but do not require advice and consent by the US Senate by a two-thirds majority). More broadly, the chapter analyses from an economic perspective, the choice between formal and informal international lawmaking (IN-LAW) by a traditional actor: the State. It is argued that the emerging picture is at best mixed. Informality differs over time and largely depends on the subject area.