In spite of the increased use of virtual worlds for language teaching and learning, few empirical studies have addressed the teachers' perspectives regarding teaching in these 3D virtual environments. An understanding of how virtual worlds such as Second Life may be beneficial for the design of enriching language learning experiences helps teachers decide if and how to best use them. The main purpose of this dissertation was to identify the competencies language teachers need in order to teach in Second Life (or a similar virtual world) and the best ways to prepare them to integrate virtual worlds into their language classes. To this end, language teachers participated in a course specifically designed to train them to use Second Life and teach in this 3D virtual environment. Their evaluation of language teachers' needs, the virtual world itself, and their experience throughout the course aided in the identification of key competencies and best practices. A case study methodology was employed in this study. Two groups of teachers were part of two teacher development courses carried out in Second Life on the topic of teaching languages in this 3D environment. Both quantitative, two Likert-scale surveys administered before and after the course, and qualitative data sources (interviews, reflective blogs, and transcripts from synchronous meetings) were analyzed. Findings indicate that, besides knowing how to use Second Life and identify its affordances and constraints, language teachers need to be able to make pedagogical decisions such as choosing an in-world place to teach and deciding how to monitor their students' work. Teachers should also consider the context, the students' needs, and the goals of the lessons to be able to create pedagogically-sound language activities in this environment. Based on their experience during the course, participants made recommendations regarding teacher preparation. Second Life's affordances and constraints pointed out by participants were the same as those identified by researchers. They could see Second Life's potential for language teaching but were concerned about the time it takes to be ready to teach in it. Taking participants' opinions and recommendations into account, a list of key competencies was created and recommendations for language teacher preparation were provided. The results of this study help shed light on this new area of research. The identification of a list of key competencies helps provide guidance for teachers interested in integrating virtual worlds into their language classes. By knowing how to use these 3D environments, teachers will be prepared to design meaningful and pedagogically-sound language learning experiences. In addition, teacher educators can use the recommendations presented here to determine the best ways to prepare teachers for this enterprise. Similarly, knowledge gained from this study is not limited to teaching in Second Life but may also extend to other similar virtual worlds. [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.]