From the time of their synthesis in the rough endoplasmic reticulum until they are secreted, packaged in lysosomes, or appear as membrane components at the cell surface, the polypeptide chains of N- and O-linked glycoproteins remain associated with intracellular membranes that are components of the secretory pathway. The various co-translational and post-translational modifications of the carbohydrate moieties of glycoproteins have been shown to occur within morphologically and functionally distinct regions of this complex membrane system. However, the sugar nucleotides, which serve as precursors to the oligosaccharide moieties of these glycoproteins, are synthesized almost exclusively in the cytoplasm. These findings raise a number of questions about the mechanisms involved in the transmembrane assembly of membrane and secretory glycoproteins. In this paper these questions are reviewed and recent studies directed towards providing answers to them arc summarized. In addition, information related to the possible role of dolichyl phosphate in regulating the glycosylation of proteins is presented.