Luminol is a presumptive test for the presence of blood. Luminol does not identify the source of the blood; rather, it suggests that a suspect pattern or stain may have originated with blood-based compounds. While luminol is a law enforcement tool often utilized during the investigation of fresh homicides, it is a very useful instrument in cold case homicide investigations. Interestingly enough, while some law enforcement agencies have used this tool extensively, others have not heard of it. This chapter presents the practical use and value of luminol in cold case investigations. Also illustrated is amido black, a general protein stain that reacts with proteins found in blood yet is not specific just for blood.