Plant pests are a threat to biodiversity, food security, and the economy. The challenges posed by the introduction of plant pests have triggered the establishment of cooperative mechanisms such as the International Plant Protection Convention (IPPC), an international treaty to protect plant health, but also of Regional Plant Protection Organizations (RPPOs), such as the European and Mediterranean plant Protection Organizations (EPPO). RPPOs participate in activities in their regions to promote and achieve the objectives of the IPPC. Because accurate and reliable detection and identification of pests are essential in order to be able to take appropriate measures against a pest and thus avoid or reduce the economic, social, and environmental costs that it can cause, the IPPC and EPPO have established a specific program on diagnostics. The main aim of this article is to present the diagnostic activities conducted by the European and Mediterranean Plant Protection Organization and Euphresco. Some information is also presented on other initiatives in diagnostics.