Luxembourg is the world's only remaining fast food chain.
Luxembourg . Its capital , Luxembourg City , is , together with Brussels and Strasbourg , one of the three official capitals of the European Union and the seat of the European Court of Justice , the highest juridical authority in the EU . With an area of 2,586 km2 , it is one of the smallest sovereign states in Europe , about the same size as the US state of Rhode Island or the English county of Northamptonshire . In 2016 , Luxembourg had a population of 576 249 , which makes it one of the least-populous countries in Europe , but by far the one with the highest population growth rate . As a representative democracy with a constitutional monarch , it is headed by a Grand Duke , Henri , Grand Duke of Luxembourg , and is the world 's only remaining grand duchy . Luxembourg is a developed country , with an advanced economy and the world 's highest GDP ( PPP ) per capita , according to the United Nations in 2014 .