Microscopy has been a key tool for cell biologists from the outset � indeed, cell biology was literally born with microscopy. This year marks the 300th anniversary of the death of Robert Hooke, whose seminal observations under the microscope (Micrographia, 1665) led to the initial coining of the term 'cell' (referring to a tiny bare room, similar to a monk's cell). Since then, microscopy has revolutionized our understanding of how cells live and die. New subcellular compartments have been discovered thanks to improving microscopy techniques, and progress in cell biology still relies in great part on advances in imaging techniques.

This past decade has seen a 'rainbow' revolution in microscopy. Fluorescent proteins, such as green fluorescent protein from the jellyfish Aequorea victoria, have been used to visualize biological processes as they happen in living cells and whole organisms. Together with improved fluorescence microscopy and time-lapse microscopy, these impressive techniques have provided insights into the dynamics of proteins and the biological processes that they regulate. Importantly, imaging techniques are now becoming available not only to specialist biophysicists but also to cell biologists — a merger that is reflected by joint conferences and collaborations that bring these communities together. Now is therefore the perfect time to focus on this central tool in cell biology.

For this reason, Nature Cell Biology and Nature Reviews Molecular Cell Biology are pleased to present this supplement on imaging in cell biology. These journals have sole responsibility for the choice and content of the supplement and, should you wish to cite any of these articles, please refer to the citation information at the end of each article, above the reference list. The supplement consists of a series of specially commissioned articles, which were selected on the basis of feedback from the research community. The topics covered in the six Review articles span both the wavelength and the resolution scale, from magnetic resonance imaging to electron microscopy, and from single-molecule to whole-organism imaging. To conclude this supplement, Roger Y. Tsien speculates on the future of imaging and the new challenges that lie ahead. We hope that the content of these articles provides not only an essential guide to the latest techniques, their advantages and limitations, but also highlights the diverse cell-biological applications of imaging techniques.

We are pleased to acknowledge the financial support of Carl Zeiss in the production of this supplement. Thanks to this support, supplement articles are available free online for six months, where they can be found together with a Focus on imaging in cell biology (see http://www.nature.com/focus/cellbioimaging). In addition to the supplement articles, this Focus site contains key imaging articles from our past issues and from other Nature Publishing Group journals, as well as movies, images and recommended links. The latter includes a link to the ongoing Nature Cell Biology and Nature Reviews Molecular Cell Biology 'Cell of the Month' competition, and some of the winning images have been used in this supplement to emphasize that cell-biological images can be not only informative, but also visually stunning.

ALISON SCHULDT, Associate Editor, Nature Cell Biology
RACHEL SMALLRIDGE, Senior Editor, Nature Reviews Molecular Cell Biology