**Preface to "Atomic Structure of the Heaviest Elements"**

Actinides exhibit a remarkable transition in terms of applied and fundamental research as they comprise the heaviest naturally occurring and fully manmade chemical elements. They have attracted the attention of atomic spectroscopists since their discovery, as it was believed that many of their elemental properties could be deduced from knowledge of their electron configuration. The tiny quantity of these elements did not prevent scientists from performing elaborate and extensive spectroscopy, such as with huge spectrographs. Thus, essential data about the atomic structure up to element 99—einsteinium—were obtained, even if some spectral lines could not be (or not correctly) assigned back then.

Today, we are much farther along in atomic spectroscopy, although actinides are far from being fully explored. In addition to better model descriptions of the atom, recent developments and advances in the field of optical spectroscopy have not only led to a better understanding of the atomic structure of the already measured elements but also to tackling the superheavy elements previously considered experimentally inaccessible.

This Special Issue of *Atoms* covers recent theoretical and experimental work about the atomic structure of actinides, as well as related topics, such as nuclear properties, transport properties in gases and the production and separation of radionuclides. With the advancing technology for production and handling of actinides and transactinides, we hope that this issue will serve as a useful resource for future work in the field of optical spectroscopy and accelerator-based laser ion sources.

> **Mustapha Laatiaoui and Sebastian Raeder** *Editors*
