Many social and economic variables are used as indicators of the level of development of countries. To determine the level of social and economic development, many criteria were studied in the fields of education, culture, health, and economy. This chapter analyzes 13 European Union countries that participated in the fifth and sixth enlargement process and Turkey, whose ongoing process of accession to the European Union is determined by using socio-economic indicators. Reference Ideal Method (RIM) is a new Multi Criteria Decision Making (MCDM) method. This method differentiates itself from other methods frequently used in literature by using the ideal points or ideal ranges. The chapter shows Turkey must improve socio-economic indicators to be close to the countries in the fifth and sixth waves of enlargement process of European Union.