In the context improving water productivity, there is a growing interest in deficit irrigation, an irrigation practice where by water supply is reduced below maximum levels and mild stress allowed with minimal effect of yield. Recognizing the potential of deficit irrigation practice, conserving scarce water resource, increasing farm productivity and enabling environmental protection this book presents the result of a number of deficit irrigation studies carried out for onion crop with a review of the impact of reduced water supplies on crop yield. The study also presents the latest research concepts and involves various practices for deficit irrigation. The overall finding in this book contributes that deficit irrigation can be beneficial where appropriately applied. Substantial savings of water can be achieved with little impact on the quality and quantity of yield.