A decision support system (DSS) is an interactive software-based system used to help decision-makers compile useful information from a combination of raw data, documents, and personal knowledge; to identify and solve problems; and to make an optimized decision. The DSS architecture consists of the database (or knowledge base), the model (i.e., the decision context and user criteria), and the user interface. The main advantages of using a DSS include examination of multiple alternatives, better understanding of the processes, identification of unpredicted situations, enhanced communication, cost effectiveness, and better use of data and resources. The application DSS in agriculture and environment has been rapidly increased in the past decade, which allows rapid assessment of agricultural production systems around the world and decision-making at both farm and district levels, though constraints exist for successful adoption of this technology in agriculture. One of the important applications of DSS in agriculture is water management at both field and district levels. Agriculture is facing more severe and growing competition with other sectors for freshwater. The water resources are becoming increasingly insufficient to meet the demand in developing countries and their quality is declining due to pollution and inadequate management. Irrigation is an effective means to enhance crop productions, but water needs to be supplied accurately, taking into account its availability, crop requirement and land size, irrigation systems, and crop productivity and feasibility. This chapter attempts to present the state-of-art principles, design, and application of DSS in agriculture, particularly irrigation practices, and to identify emerging approaches and future direction of research in this field.