This study examines “internationalization process” in detail. After defining and giving a brief history of internationalization concept, reasons, which push firms into internationalization, are discussed according to economics and marketing disciplines. “Foreign Direct Investment Theory”, “Stage Models”, and “The Network Model” which arc the three basic approaches for internationalization in literature are also mentioned. Besides, due to its importance nowadays, internationalization for service firms is studied at the end of that paper. It can be said that exportation is the first stage of internationalization process and firms pass to the next higher-level stages on the internationalization process depending upon their experience based on their exportation efforts. In addition, firms size is not too important characteristic on the internationalization especially in exportation.