Science diplomacy emerged in the early years of the 21st century as a new vocabulary and a new concept in international relations, although the practice of science diplomacy has deep historical roots and various forms that were not labeled as such before. Science diplomacy refers to professional practices at the intersection of the world of science and that of diplomacy. It is also a subject of study that gives rise to a scholarly literature. Basically, the rationale of science diplomacy is twofold: advancing a country’s national interest and addressing global challenges. Science diplomacy encompasses a great range of activities to promote and secure a state’s foreign policy objectives and of activities to secure global public good at the transnational level, such as using scientific advice and expertise, enabling international scientific cooperation, bringing scientists on board of diplomatic negotiations, or appointing science attachés to embassies. International scientific cooperation is sometimes confused in the discourse with science diplomacy. However, if scientific cooperation is possible only with diplomatic assistance, serves a nation-state’s foreign policy objectives, promotes national interests, or aims to address global issues, then it is science diplomacy. Otherwise, it is not. Science diplomacy is also closely related to a state’s political system and beliefs because the effective use of science diplomacy contributes a great deal to a state’s power and influence in world politics and in international relations, and it helps to generate soft power of attraction and cooperation. A few notable institutions are active in science diplomacy, promote international dialogue on global issues, disseminate practices, and take part in the debate of the science diplomacy concept. They include the Center for Science Diplomacy of the American Association for the Advancement of Science (AAAS), the International Network for Government Science Advice (INGSA), and the Science Diplomacy Center of Tufts University, and multilateral scientific organizations, such as the International Institute for Applied System Analysis, the International Science Council, and the Science Diplomacy Thematic Network at the University of the Arctic. National and international academies of sciences sometimes intervene in this debate. Professional literature on science diplomacy is abundant and academic literature is growing as well, which has not led, however, so far to the emergence of a genuine theory of science diplomacy. This article aims to guide readers in their comprehension of science diplomacy and of the related debates through a selection of sources that shed light on science diplomacy both in theory and in practice from various viewpoints.