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 pi 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.