Fluorophores are chemical compounds that can re-emit light upon light excitation and typically contain several combined aromatic groups, or plane or cyclic molecules with several π bonds. In this book, the authors present current research in the study of the characteristics, synthesis, and applications of fluorophores. Topics include the application of fluorescent molecular probes in cell biology; fluorescent probes based on upconverting lanthanide nanostructures; aromatic hydrazine-based fluorophores synthesis, spectroscopy and bioapplications; and the concentration dependence of humic acid fluorescence intensity.