One of the major challenges of modern biology and medicine consists in finding means to visualize biomolecules in their natural environment with the greatest level of accuracy, so as to gain insight into their properties and behaviour in a physiological and pathological setting. This has been achieved thanks to the design of novel imaging agents, in particular to fluorescent biosensors. Fluorescence Biosensors comprise a large set of tools which are useful for fundamental purposes as well as for applications in biomedicine, drug discovery and biotechnology. These tools have been designed and engineered thanks to the combined efforts of chemists and biologists over the last decade, and developed hand in hand together with imaging technologies. This volume will convey the many exciting developments the field of fluorescent biosensors and reporters has witnessed over the recent years, from concepts to applications, including chapters on the chemistry of fluorescent probes, on technologies for monitoring protein/protein interactions and technologies for imaging biosensors in cultured cells and in vivo. Other chapters are devoted to specific examples of genetically-encoded reporters, or to protein and peptide biosensors, together with examples illustrating their application to cellular and in vivo imaging, biomedical applications, drug discovery and high throughput screening.