Editorial overview: Molecular imaging.

Optical microscopy is a very popular technique for investigating the living cell. Especially, far-field illumination in combination with fluorescence detection of tagged molecules such as proteins offers minimal invasiveness yet large specificity and sensitivity. Unfortunately, its full applicability is still limited due to several reasons. For example, introduction of a fluorescent label may still disturb the system or its information content may be biased by limited signal due to, for example, photobleaching. In the context of the latter case, one has to keep in mind that the illuminating light might introduce phototoxic effects on the system under study. Therefore, great efforts are undertaken to improve the labeling characteristics and minimize illumination stress. Yet, the largest limitation of far-field microscopy is probably its limited spatial resolution. Owing to diffraction, light cannot be focused to an infinitely small spot, which for visible light amounts to a lower limit of about 200 nm. As a consequence of this limitation, similar objects that are closer together than 200 nm cannot be discerned and details of, for example, cellular protein distributions can be visualized only to a certain extent. Fortunately, recent years has seen the development of superresolution far-field fluorescence microscopy or nanoscopy techniques such as STED, GSD, RESOLFT, (f)PALM, (d)STORM, GSDIM, etc. (see e.g., [1–9]). All fluorescence nanoscopy techniques in one way or the other apply the reversible transition between states of different fluorescence emission characteristics (e.g., between a dark and a bright state), thereby ensuring that the measured signal only stems from molecules within a region of the sample that is much smaller than 200 nm, thus reaching molecular-scale imaging of cellular components. Efforts in correlatively combining optical nanoscopy with other microscopy tools such as electron microscopy will thereby significantly increase the information content. The enclosed range of articles gives an update on recent advances in labeling technology, reduction of phototoxicity, applicability of optical nanoscopy as well as combination with electron microscopy.