Using Desimone's (2009) framework for studying the effects of teacher professional development (PD), this qualitative study examines the effectiveness and impact of a 10-week professional development course on 31 foreign language (FL) instructional faculty and staff in a U.S. Government language school. Data sources included 31 K-W-L reflections, 15 questionnaire responses, and nine semistructured interviews from course participants. Etic and emic analysis revealed evidence of Desimone's framework as well as emerging themes that showed the importance of modeling, expanding the professional community, confirming beliefs, increasing confidence, and developing a learner-centered mindset to effective PD for FL teachers. These emerging themes are presented as an expansion of Desimone's core conceptual framework based on contextual factors specific to PD for FL instructors of adult learners. The results of this study can inform the development and evaluation of FL teacher PD and ground future studies in this context using Desimone's framework.