Periodic Acid-Schiff (PAS) is a histochemical staining technique used to detect polysaccharides, such as glycogen and mucosubstances, in tissue samples. The method involves treating the tissue with periodic acid, which oxidizes the carbohydrates, followed by the application of Schiff's reagent. This results in a bright magenta color, indicating the presence of these substances. PAS staining is commonly used in pathology to identify various conditions, including glycogen storage diseases and fungal infections. It helps pathologists visualize and diagnose abnormalities in tissues, making it a valuable tool in medical research and clinical diagnostics.