The development of new drugs requires increasingly the performance of large multinational clinical trials (MCT) with a common protocol. They must be planned when the demonstration of a hypothesis, which requires specific conditions (for example availability of patients with rare diseases, a particular infrastructure or expert knowledge in trial centers) has to be proven in an acceptable time. Our own experience has shown that such multinational trials are more time-consuming in their preparation and their analysis than multicenter trials which are run in one country. MCTs are associated with complex problems due to many differences in medical culture, treatment strategies, administrative guidelines, etc., between countries. When possible MCTs should be realized in countries and centers with relatively similar medical practices. A global coordination is necessary to control the progress of the trial in the different countries. The major requirements for the successful realisation of an MCT, from the writing of the first draft of the protocol until the publication of the results, are a well-coordinated multidisciplinary team and an effective project management.