This article describes the scientific objectives, the scientific and operational plans and the observational programme of the international Atlantic Tropical Experiment in which some 70 countries pclrticipated during Jun-September 1974. The main aim waa to make, for the first time, an intensive study of the structure, evolution and transport properties of tropical weather systems and their role in the global circulation of the atmosphere as a whole. Some preliminary results are described but it will be two or three years before the vast mass of data is fully processed and analysed and made available in the form of validated data sets for detailed researoh studies and for input to numerical models. However, there is already good reason to believe that the major scientific objectives of the experiment will be achieved and that the knowledge gained will lead to improved weather forecasting both in the tropics, and in middle latitudes where accurate predictions for more than about three days ahead may not be possible without taking into account the import of heat and moiature from the tropical oceans through the large convective clouds of the tropical atmosphere.