This work presents and critically evaluates attempts to theorize and conceptualize public diplomacy within several disciplines, including international relations, strategic studies, diplomatic studies, public relations, and communication. It also examines research methods used to investigate public diplomacy, including models, paradigms, case studies, and comparative analysis. The work identifies promising directions as well as weaknesses and gaps in existing knowledge and methodology and outlines a new research agenda. The presented analysis and examples suggest that only a systematic multidisciplinary effort and close collaboration between researchers and practitioners can lead to a coherent theory of public diplomacy.