Fluorescent dyes, also known as fluorophoresFluorophores, are chemical compounds that absorb light at specific wavelength and emit light at a longer wavelength, which can be utilized in a vast number of applications including microscopy, medical diagnostics, flow cytometry and material science. The types of fluorescent dyes, include organic dyes, quantum dyes, protein-based dyes, and inorganic fluorophores. This chapter focuses on organic dyesOrganic dyes, which are characterized by an extended (conjugated) electron system that can easily emit light after excitation with electromagnetic radiation. The size of the conjugated electron system allows to tailor the fluorescent dyes to different excitation sources, covering the electromagnetic spectrum from UV light to the near infrared (NIR) region. Introduction of hydrophilic, charged groups, or suitable polymers like polyethylenglycole allow for better biocompatibility. Coupled to antibodies, proteins, oligonucleotides, or DNA fragments, they are used in a variety of imaging methods, which make them indispensable in modern bioanalytics today. With ongoing advancements, new dyes with enhanced properties continue to be developed for emerging technologies and applications.