β-1,4-Galactosyltransferase 7 (β4GalT7) is a key enzyme in the synthesis of two classes of glycosaminoglycans (GAG), i.e., heparan sulfate (HS) and chondroitin/dermatan sulfate (CS/DS). GAG chains are linear polysaccharides of alternating hexuronic acid and N-acetylhexosamine residues, commonly linked to core proteins to form proteoglycans with important roles in the regulation of a range of biological processes. The biosynthesis of GAGs is initiated by xylosylation of a serine residue of the core protein followed by galactosylation, catalyzed by β4GalT7. The biosynthesis can also be initiated by xylosides carrying hydrophobic aglycons, such as 2-naphthyl β-D-xylopyranoside. We have cloned and expressed β4GalT7, and designed a cell-free assay to measure the activity of this enzyme. The assay employs a 96-well plate format for high throughput. In this chapter, we describe the cloning, expression, and purification of β4GalT7, as well as assays proposed for development of substrates for GAG priming and for investigating inhibitors of β4GalT7.