Greece is a unitary parliamentary republic, and a developed country, with an advanced high-income economy, and a high quality of life, ranking simultaneously very high in the Human Development Index. Its economy is the largest in the Balkans, where it is an important regional investor. A founding member of the United Nations, Greece was the tenth member to join the European Communities (precursor to the European Union) and has been part of the Eurozone since 2001. It is also a member of numerous other international institutions, including the Council of Europe, the North Atlantic Treaty Organization (NATO), the Organisation for Economic Co-operation and Development (OECD), the World Trade Organization (WTO), the Organization for Security and Co-operation in Europe (OSCE), and the Organisation internationale de la Francophonie (OIF). Greece's unique cultural heritage, large tourism industry, prominent shipping sector and geostrategic importance classify it as a middle power.