**Preface to "Progress in Power-to-Gas Energy Systems"**

Like many of you, dear readers, I plunged into the exciting field of hydrogen and power-to-gas, which promises us renewable energy storage and use in molecular forms—either gaseous or liquid—to decarbonise all energy sectors that currently depend on fossil fuels. Over the past decade in this topic, I have been privileged to witness a number of developments, trends and advancements that have enabled us to stand where we stand today. We find ourselves at a crossroads between theoretical groundwork and the actual, large-scale technical implementation of the energy transition.

This reprint brings together current research findings from highly diverse disciplines, all of which can make a valuable contribution to the success of the energy transition. I am convinced that this interdisciplinarity of the common global challenge and its cross-sectoral character are central challenges of the energy transition. There will not be one blueprint solution that can be applied across the globe; rather, the research results yield insights for possible partial solutions that can be implemented in national, regional or even local energy systems at different implementation levels and detail, depending on boundary conditions and specific demands.

The first contribution in this reprint is an editorial regarding the contents of the ten following research articles. It contains a table of the articles, including research fields, titles and methods. The reader can use it as a quick overview before turning to the detailed articles. I can therefore be brief and simply refer to the overarching themes covered here: You will find three contributions regarding combustion research, one contribution regarding electro catalysis; several contributions from the perspective of energy economy; articles on energy storage, the mobility sector, and several energy system analyses; and finally, a contribution on energy law and regulation.

As Guest Editor, I have been able to rely on competent co-authors, all of whom are recognised experts in their field and represent leading research institutions. The scope of this reprint can in no way do full justice to their wealth of knowledge and experience, but it offers exciting insights into their latest research findings. My sincere thanks go to them once again. Special thanks are also due to all colleagues not listed as co-authors in this reprint. Thank you for the lessons I was able to learn from working with you. It is my pleasure to share the results with interested readers worldwide.

> **Johannes Schaffert** *Editor*
