This volume is an admirable collection of manuscripts, offering the reader a valuable and accessible overview demonstrating the broad applicability of FRET (Förster resonance energy transfer). The applications encompass a large range of topics from single molecules to measurements in tissue, with a main emphasis on FRET imaging. The topics have been carefully chosen to impart not just a comprehensive synopsis and up to date highlights, but also bring to light many technical and fundamental aspects that are indispensable for practitioners.

The FRET process is familiar to scientists in many diverse disciplines including biology, chemistry biochemistry, biotechnology, bioengineering, chemical engineering, and physics. There are few acronyms in science that are so widely recognized, and this alone is a tribute to the importance and usefulness of FRET techniques. This technique, based in quantum mechanics, continually inspires new applications in a wide range of research. One of the major goals of all physical, chemical and biological sciences is to understand phenomena on a molecular scale; and this requires information about the spatial relationships between molecules. FRET is usually the best, and often the only, way to measure quantitatively distances between molecules between 0.2 to 20 nanometers, or simply to determine whether two molecules interact within this distance.

Jean and Francis Perrin proposed over 75 years ago that dipole-dipole interactions could explain the well-known experimental fact that chromophores can interact without collision at distances greater than molecular diameters. Twenty years later Förster provided a quantitative explanation for the non-radiative energy transfer. In a few seminal papers he laid the groundwork for all later theoretical extensions, which provide the basis for modern applications of FRET to determine accurate intermolecular distances, and distance differences. Most importantly, Förster presented a theory that makes it possible to extract the relevant information from simple experimental data that allows the quantitative analysis of molecular scale distances. Förster showed us how to relate the molecular scale electronic Coulombic coupling between the donor and acceptor chromophores to the transition dipoles of the interacting molecules. This allows the efficiency of transfer to be expressed simply in terms of an overlap integral, the index of refraction, the quantum yield of the donor, the dipole-dipole interaction angle (kappa) and fundamental constants. In general, the only unknown is the distance between the chromophores, which is usually the parameter sought. The overlap integral and the quantum yield are calculated from simple experiments on the separate chromophores. The kappa parameter can usually be adequately approximated, or the error estimated. Thus, the original physical insights of Förster continually inspire deeper theoretical analyses of energy transfer that extend its applicability to new experimental situations.

FRET is the most commonly applied experimental measurement for estimating distances between molecules in solution, and it has been more recently extensively applied in optical microscopy. New experimental techniques measuring FRET efficiency, and more efficient and accurate methods for data analysis, are continually being developed that are applicable in new experimental environments. Because the molecular information lies in the spectroscopic signal, FRET can be applied to macroscopic samples as well, even to whole organisms. The titles of this edited volume give a good indication of the broad applicability and up-to-date applications of FRET. We thank Ammasi Periasamy and Richard Day, the editors of this excellent volume, who have undertaken the editorial job of assembling chapters from internationally recognized experts, in order to give interested readers an extensive, insightful overview of the wide range of FRET applications, particularly in the biological and imaging areas. Although lively research is being pursued in other scientific areas, it is particularly in biological research and biotechnological applications that FRET has been most profoundly exploited in the last few decades.

Robert M. Clegg, University of Illinois at Urbana-Champaign