Increasingly, contemporary mental health services research projects aim to combine qualitative and quantitative components. Yet researchers often lack theoretical and practical guidance for undertaking such studies. In September 2006 the authors convened under the auspices of the National Institute of Mental Health at a working conference, "Mixed Methods in Community-Based Mental Health Services Research." This meeting provided the opportunity for participants to share their experiences in conducting mixed-methods research, to critically consider problems they had encountered and their solutions, and to develop guiding principles for others conducting similar research. The authors' discussions, which are described in this article, emphasize that the problems encountered by mixed-methods research teams are rarely simple misunderstandings but often reflect epistemological differences that are overlooked in the study planning phases. Failure to acknowledge these different worldviews may result in significant tensions between members of the study team, use of qualitative methods that are insufficient or inappropriate for a particular research question, or serious conflicts when team members belatedly discover they are interpreting key concepts -- or each other's research techniques -- differently. The authors conclude that ongoing communication is the organizing principle for robust and effective mixed-methods research. Among the recommendations for preventing problems are collaboration between quantitative and qualitative researchers during the study design phase; open acknowledgement of the philosophical approaches brought to the study by various team members; and because not all challenges can be anticipated, a shared willingness to negotiate emerging problems.