Transgenic plant production has been intimately connected to the beta-glucuronidase (uidA or GUS) gene used as a reporter marker gene. The enzyme stability and the high sensitivity and amenability of the GUS assay to qualitative (histochemical assay) and to quantitative (fluorometric or spectrophotometric assay) detection are some of the reasons that explain the extensive use of uidA gene in plant genetic transformation. Methods for uidA gene detection have been thoroughly described in the literature. The aim of this chapter is to describe the basic protocols needed for GUS detection in a plant genetic transformation laboratory.