The present study explores the implementation of district heating and cooling systems across a broad set of case studies reported in the literature. Topics addressed include their history, system identification, energy sources, design considerations, environmental impact, economic feasibility, performance analysis and the role of energy policy. The history of district heating and cooling systems reveals how available technology has influenced the configuration of district energy systems as more efficient and cleaner methods of providing heating and cooling have arisen. This leads to system identification based on primary energy sources, including the deployment of more and more renewable energy streams. Advantages and disadvantages of each energy source are examined in detail. Policies created by government and international entities will have a major impact on the future of research and development in district energy systems. Incentives may become necessary for creating favorable conditions for the efficient construction and utilization of district heating and cooling. Outcomes of these policies influence design considerations underlying any district energy system and their contribution in sustainability. Studies on greenhouse gas emissions along with the economic impacts of district energy construction are part of the design process and optimization of district energy systems should include economic and environmental considerations and not solely thermal efficiency. District heating and cooling systems are often integrated with components such as absorption chillers, cogeneration and thermal energy storage. Performance analysis using exergy and energy analysis have revealed several sources of irreversibility in district heating systems with these elements. If understood properly, these can greatly enhance system operation. Awareness and accommodation of the many factors discussed in this paper can improve the soundness of any district heating or cooling installation. Published by Elsevier Ltd.