This longitudinal study focuses on the language production of two siblings, aged 6 and 9 at the beginning of the data collection period, who have been brought up in a bilingual family in New York. The parents of the two girls are native speakers of German and Spanish, respectively, and English for them is the language of education and the larger community. The study specifically examines the phenomenon of code switching, or transitioning between languages, either within one sentence or within one speech situation. I examine the extent to which these switches are caused by deficiencies in vocabulary in a specific language, and seek to identify other possible causes for such transitions. The data collected mainly through recording and transcription of the children's speech within the family home allowed me to identify a number of sociopragmatic functions most commonly fulfilled by producing mixed utterances, such as referring to a specific person, including or excluding someone from the conversation, changing the interlocutor or the topic or explaining or insisting on a certain idea. Lexical need was also an important cause of code switching, although it did not prevail over the other categories. The distribution in the amount and function of code switches turned out to be in a dynamic state, with both quantitative and qualitative changes observed throughout the study period. The age difference between the children and the relationship between the younger and elder sibling were additional factors which influenced their language choice. I conclude that code switching, especially in the case of child speech, should be considered a fluid and multifaceted phenomenon which represents the speaker's role in the conversation and reflects multiple social and pragmatic functions; while elements of two (or more) languages are often combined for purely lexical reasons, this is only one aspect of trilingual code switching, which allows the speaker, consciously or not, to explore the three languages as ways of establishing his or her personality and looking at reality both within and outside the means of each particular language. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]