Water and sanitation played a crucial role in the evolution of Portugal and its empire, which in the sixteenth century dominated large portions of the world. Two relevant civilizations, the Roman Empire and the Arab invasion, had great influence on Portugal's water and sanitation knowledge. Following the creation of Portugal in AD 1143, the Cistercian order was called for removing Arab influence and received large domains, where it built large monasteries, all provided with remarkable examples of water supply, sanitation and waste management, merging the Roman background in sanitary engineering with the local Arab experience. One of them, the Monastery of Christ in Tomar, is provided with a brilliant water and waste self-sustainable system, based on rainwater collection and storage, wastewater treatment and application in agriculture of treated waste and effluent. It testifies to the experience and innovative expertise of the Cistercian Order in sanitary/hydraulic engineering. It shows also one of the world's first examples of a wastewater treatment plant. Their knowledge influenced also the lifestyle and water management of Portuguese medieval cities, ruled by municipalism, and protected the population from pestilence until the first half of the fourteenth century, making Portugal a powerful country, in contrast with the rest of Europe.