The ever increasing popularity of virtual worlds, also known as 3-D multi-user virtual environments (MUVEs) or simply virtual worlds provides language instructors with a new tool they can exploit in their courses. For now, "Second Life" is one of the most popular MUVEs used for teaching and learning, and although "Second Life" was not initially designed as an educational tool, it has developed into one used in higher education institutions worldwide. In the field of second language acquisition, "Second Life" could be used to immerse students in the target language and culture. Students can travel to virtual versions of cities or countries where the target language is spoken and engage in meaningful interactions while instructors can track written and spoken linguistic patterns through the program's features. Virtual worlds appeal to language instructors and computer-assisted language learning (CALL) researchers alike because of the affordances it offers. Yet, there are many areas left to explore. The focus of this study was to measure and evaluate the effects of using the 3-D MUVE "Second Life" on students' motivation to learn Spanish as a foreign language, and on students' academic achievement and language proficiency. This study incorporated qualitative and quantitative research methods to assess the effects of the treatment (i.e., using "Second Life") on each of the variables. Significant differences existed in motivation between the groups; however, academic achievement and language proficiency between the control and experimental groups were very similar. The researcher used a phenomenological approach to gather and analyze data from student and instructor interviews. This approach helped the researcher understand the effects of using "Second Life" in a Spanish course by providing insight on students and the instructor's opinions about the virtual world's applicability as a tool for language learning and teaching. Circumlocution, exposure to the target language, finding people and destinations, and technical difficulties, such as the use of voice chat were salient themes during the interviews. Future efforts should focus on improving the design process of activities through the application of instructional design principles to ensure that they provide sufficient guidance and scaffolding for students to develop their language skills. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]