Technical Abstract: The tetrazolium test is a biochemical viability test for seeds. Hydrated and cut or pierced seeds are exposed to a solution containing the compound 2,3,5 triphenyl tetrazolium chloride (TTC). Initially colorless, TTC is converted to formazan red in the presence of live tissue. Seed analysts examine the stained tissues and determine if the seed is viable or nonviable. History, current status, seed preparation methods, evaluation, benefits and limitations of the test are discussed. Photos of embryo types and stained seeds from 23 families important in ornamental plant commerce are provided.