Poly-N-acetyllactosamine structures occur in mammalian glycoproteins in both N- and O-linked glycans. They represent a backbone for additional modifications by fucosyltransferases, sialyltransferases and sulfotransferases. These glycans have been suggested to be involved in biospecific interactions with selectins and other glycan-binding proteins. Moreover, the poly-Nacetyllactosamine chains in N-glycans have been found to promote tumor progression and metastasis. Poly-N-acetyllactosamine chains are synthesized by repeated alternating additions of Nacetylglucosamine and galactose, catalyzed by β-1,3-N-acetylglucosaminyltransferases (poly-N-acetyllactosamine synthase) and β-1,4-galactosyltransferases. This review describes the poly-N-acetyllactosamine assembling machinery and focuses on recent advances in the molecular cloning and characterization of poly-N-acetyllactosamine synthase gene families. Recent progress in revealing the biological functions of poly-N-acetyllactosamine structures by various approaches in vitro and in vivo using different model systems has also been summarized.