This chapter, coauthored by a community psychologist and an anthropologist who has contributed to community psychology, reviews the relevance of other disciplines to our field and why transdisciplinary work is critical in moving community psychology closer to its fundamental mission: reducing the social, structural, and psychological factors that prevent psychological well-being. Community psychology recognizes local communities as critical and complex social systems in which internal and external forces converge at multiple levels to influence human behavior and wellness. In this chapter we review the interaction of community psychology with other social sciences and applied fields. We link this review to a conceptualization of community psychology as a transdisciplinary field that can bring complex theories, methods, results, and interventions to bear on reducing injustices in local communities.We begin with an analysis of the major social science disciplines by levels of analysis, followed by a brief review of definitions of multidisciplinary, interdisciplinary, and transdisciplinary modes of collaboration. We present a model for ecological (multilevel, transdisciplinary, dynamic) community research and action, with special attention to issues of power and social justice. We then delve into historical and recent contributions to community psychology made by ideas from public health, epidemiology, ecology, natural biology, and environmental studies; sociology; anthropology/ethnography; community organizing and development; organizational studies; 1 Despite their importance, fields that have had a primarily methodological influence (e.g., statistics) are not covered, because our main interest is in disciplines that have had a broader conceptual or topical influence. We also do not review the influence of other branches of scientific or applied psychology, which are covered elsewhere in this handbook.