**Preface to "District Energy System Design"**

The integration of energy efficiency into the urban planning process is paramount in view of the current context of energy and environmental transition. However, reducing the energy footprint at the district level is a new approach that must be complemented by specific developments. Thus, the contributions related to the research of the best energy concepts cover three main topics: the simulation, the optimization procedure and finally the decision-making method. Towards this end, dynamic simulation tools based on physical models (occupant, building, production, networks, etc.) at the urban scale must be developed in order to represent the behavior of all energy flows of a district integrated in its environment. Then, specific characteristics of the problem complexity should be studied by means of a multi-objective and multi-stage optimization procedure. This cross-cutting approach could combine energy, economic and environmental aspects in order to create configurations that guarantee the best overall performance. Lastly, the selection of the preferential action could be achieved by the use of multi-criteria analysis methods that provide the planners with all the data that they need, enabling them to make the choice that best meets their expectations. This Special Issue includes five articles focused on these topics. The subjects covered are related to demand-side management, thermal flexibility, distribution network planning, multi-criteria decision-making, optimization of meshed distribution network, multi-carrier energy systems, combined optimal design, seasonal thermal energy storage, generation district heating networks, thermal energy storage and vehicle-to-grid for peak shaving, among others. All the papers in this Special Issue have been peer reviewed and subjected to the editorial standards of Energies. All our warmest and most sincere thanks go to all the authors for their timely response and careful revisions. They have produced very valuable contributions and provided high-quality materials for this Special Issue. Finally, we would like to sincerely thank the anonymous referees for their voluntary work and expert review.

**Christian Inard, J´er ˆome Le Dr´eau**

*Special Issue Editors*

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