Fluorescent in situ hybridization (FISH) is a molecular technique used for the detection of specific DNA sequences within the chromosome. It relies on the complementary binding between the fluorescently labeled probe and the target sequence. This paper describes how this method was first developed, and the basic principle and the procedure behind it. Furthermore, it covers the basic applications of FISH, including its use in microbiological diagnostics, diagnosis of solid tumors, diagnosis of hematological malignancies, evaluation of sperm and diagnosis of DiGeorge syndrome, along with its applications in plants.