In spite of widespread consumption of coffee in Europe at the time of the Royal Danish expedition to Arabia 1761-1767, little was known of the cultivation of coffee in Yemen and of the Arabian coffee export to Europe. Fresh leaves of qat were used as a stimulant on the Arabian Peninsula and in East Africa, but before the Royal Danish expedition to Arabia this plant was known in Europe only from secondary reports. Two members of the expedition, Carsten Niebuhr and Peter Forsskål, pioneered studies of coffee and qat in Yemen and of the Arabian coffee export. Linnaeus’ instructions for travellers requested observations on the use of coffee, but otherwise Forsskål and Niebuhr’s studies of coffee and qat were made entirely on their own initiative. Now, 250 years after The Royal Danish expedition to Arabia, coffee has become one of the world’s most valuable trade commodities and qat has become a widely used and banned drug.