Biotin-avidin detection systems are widely used in both immunocytochemistry and molecular biology. They take advantage of the high affinity of biotin, a low-molecular-weight vitamin, for avidin, an egg-white protein. Because of the problem of nonspecific binding of avidin, streptavidin has largely replaced avidin for immunocytochemical procedures. Streptavidin/colloidal gold-biotin detection systems for electron microscopy are most commonly used in postembedding immunocytochemistry. Usually, the primary antibody is unlabeled, the secondary antibody is biotinylated, and the colloidal gold is conjugated to streptavidin. In certain applications, the primary antibody may be biotinylated and no bridging antibody is needed. This chapter details the use of streptavidin-gold for postembedding labeling.