In this dissertation based on empirical studies conducted by the author and reprinted as a series of articles in the text, Kuure studies the development of different types of bilingualism. The primary purpose of the studies is to determine the significance of age in the acquisition of a second language. Evidence is also provided for describing the acquisition and use of languages in various multicultural and multilingual settings. Data for the studies were obtained from Finnish-Swedish bilingual students between the ages of 13 and 15 for whom Swedish was the second language.