The direct labeling of nucleic acid probes with horseradish peroxidase (HRP) was first described by Renz at EMBL in 1984 (1 ). The methodology was combined with enhanced chemiluminescence (2 ) (a light producing HRP catalyzed reaction; see chapter 20 ) allowing the detection of specific hybrids on membranes (3 ). Further development led to the availability of the first light-based nucleic acid detection system; this was capable of detecting 2.5 pg of target nucleic acid on genomic Southern blots (4 ). Subsequent protocol and reagent improvements now enable researchers to reliably detect 0.5 pg of target nucleic acid.