Publisher Summary This chapter focuses on the microsomal sulfation of proteochondroitin, chondroitin, and chondroitin oligosaccharides. Chondroitin sulfate—a polymer of gIucuronic acid (GlcA) and N-acetyl galactosamine (GalNAc)—can be sulfated on either or both the 4 and 6 positions of the GalNAc and on the 2 position of the GlcA. Chondroitin sulfate is found in most—if not all—tissues and cells, and chondroitin sulfotransferase activities have been observed from multiple sources. Nascent proteochondroitin at the microsomal site has been shown to be the best substrate for these microsomal sulfotransferases, but exogenous proteochondroitin, chondroitin, and chondroitin oligosaccharides can also be utilized. The efficiency of sulfation with these exogenous substrates is lower than that with endogenous nascent proteochondroitin. The assay methods utilizing endogenous and various exogenous substrates are also described in this chapter.