This volume is focused on one of the most important challenges in sensing and imaging technologies: the design of fluorescence reporters with advanced properties. Here organic dyes occupy leading positions, in tough competition with novel materials such as metal chelating complexes and semiconductor nanoparticles. 11 chapters written by top experts in the field show new possibilities in the design of organic dyes as fluorescent labels and reporters. They particularly highlight the progress that has been made in enhancing the response to intermolecular interactions and their excited-state reaction dynamics (intramolecular charge and proton transfers), and on the development of dyes with strong two-photon absorption and emitting in the near-IR region. Furthermore, fluorophores incorporated into new members of the green fluorescent protein family, an invaluable tool for live cell imaging, are examined.