**4. Comments**

At the 13th Sustainable Development of Energy, Water, and Environment Systems Conference held in 2018 in Palermo, Italy, interesting topics concerning sustainable development and energy efficiency were discussed and presented. This SI presents some of this work within the following main topics: energy efficiency and energy policy in urban areas, energy efficiency in industry and biomass and hydrogen. The editors of this SI of *Energies*, dedicated to 13th SDEWES Conference, believe that the above reported papers are focused on topics considered pivotal for the *Energies* journal. Such studies can be considered as fundamental and potentially practical tools to promote and disseminate the energy sustainability in different sectors: industries, metropolitan and urban areas, waste heat recovery, waste materials, water-energy nexus, etc.

Information on the future SDEWES Conferences and related activities are available on the website of the International Centre for Sustainable Development of Energy, Water, and Environment Systems (SDEWES Centre).

**Author Contributions:** F.C. and M.V. prepared an initial draft of the manuscript, which was completed, corrected and reviewed by M.C., N.D., Q.W. and P.A.Ø.

**Funding:** This research received no external funding.

**Acknowledgments:** The guest editors thank the authors of the papers that submitted their manuscripts to this special issue for the high quality of their work. We also thank all the reviewers who provided valuable and highly appreciated comments and advice, and the managing editors of *Energies* for their patience and excellent support.

**Conflicts of Interest:** The authors declare no conflict of interest.
