**Preface to "Duality Symmetry"**

I would like to warmly thank all the authors and reviewers of this Special Issue for their contribution to the process. This Special Issue is composed of four Articles, one Review and one Perspective. The context of the contributions ranges from string theory to applied nanophotonics, which, as anticipated, shows that duality symmetries, and electromagnetic duality symmetry in particular, are useful in a wide variety of physics fields, both theoretical and applied. Moreover, several of the contributions show how the use of symmetry arguments and the quantification of symmetry breaking can successfully guide our theoretical understanding and provide us with guidelines for system design. I fervently anticipate the practicality and future outcomes of such research avenues.

> **Ivan Fernandez-Corbaton**

> > *Special Issue Editor*
