The last few years have seen an upsurge of interest in the study of cells by optical microscopy. The advent of new techniques such as confocal microscopy and the availability of extremely sensitive digital imaging devices are revolutioniz-ing the field. A number of groups have developed new ways of making spectroscopic measurements at the microscopic level, accompanied by the introduction of appropriate sen-sor molecules for cellular assays. The aim of this volume will be to bring together the various advances in order to provide the reader with an up-to-date account of what can now be achieved with modern optical microscopic methods.