This chapter lists the CAS registry number, chemical structure, chemical abstract (CA) index name, other chemical names, Merck Index number, chemical/dye class, synthesis procedure, molecular formula, and molecular weight of the Propidium Iodide (PI). Other properties listed for this fluorescent dye include the physical form, solubility, melting point, absorption maxima, emission maxima, and molar extinction coefficient. The chapter also includes references on the use, properties, and safety/toxicity of Propidium Iodide (PI). It presents a list of imaging/labeling applications, biological/medical applications and industrial applications of Propidium Iodide (PI).