Question: What are some of the forces shaping the evolution of medical libraries, and where might they lead? Data Sources: Published literature in the fields of library and information sciences, technology, health services research, and business was consulted. Main Results: Medical libraries currently have a modest footprint in most consumers' personal health ecologies, the network of resources and activities they use to improve their health. They also occupy a relatively small space in the health care, information, and business ecologies of which they are a part. Several trends in knowledge discovery, technology, and social organizations point to ways in which the roles of medical libraries might grow and become more complex. Conclusion: As medical libraries evolve and reach out to previously underserved communities, an ecological approach can serve as a useful organizing framework for the forces shaping this evolution.