In 2014, the University of Maryland Libraries adopted a Subject Liaison Librarian model to proactively deliver a full range of services to its faculty and students and move the Libraries into the future. Subject liaison librarians are Public Services librarians, responsible for developing and managing collections, reference, instruction and outreach in their assigned disciplines. They are required to be recognized specialists in their subject areas, and at the same time be professional librarians, skillful in library-related operations. Challenged by severe changes in the academic environment, rapidly changing technology, the proliferation of information, and consistently difficult budgets, subject liaison librarians must continually improve their skills and knowledge to keep current with the needs of the campus community, library profession, and their subject areas. To this end, the University of Maryland Libraries developed a professional development program for their subject liaison librarians, which combines internal and external learning and training opportunities, both individual and group based. This paper will discuss the professional development program for subject liaison librarians at the University of Maryland Libraries. Note: subject liaison librarians are being also referred to here as liaison librarians, or subject librarians.