The 2015 Halley Lecture delivered by Professor Peter J. Webster Each year the monsoons bring rainfall to nearly half the population of the planet. Small variations in monsoon rainfall can lead to flood or drought, feast or famine. Therefore, explaining the physics driving the monsoon and turning this knowledge into predictions is one of the great problems in science. In 1686 Sir Edmund Halley, with trade and navigation on his mind, suggested that the monsoon was driven by the buoyancy induced by the differential heating between the Indian Ocean and the landmass of South Asia. With a few embellishments, such as noting the importance of the rotation of Earth, his theory has stood the test of time. However, during the last 20 years, advances in our understanding of global fluid dynamics, suggest that a land-sea heating contrast is not sufficient. In fact, at the same latitudes of maximum monsoon summer rainfall, in other parts of the world there are deserts. Here we will develop an alternative, albeit simple, general theory of the monsoons and discuss how this may be translated into useful predictions and a greater understanding of how the monsoons will fair in a changing climate.