The discovery, development, production and clinical application of recombinant glycoproteins for therapeutic administration in humans has been, and continues to be, an area of intensive scientific and medical effort. This effort has engendered considerable interest in the biological and therapeutic implications of post-translational modifications, particularly the most elaborated and sophisticated of these, protein glycosylation. As a result, numerous studies have appeared in the literature, especially within the past few years, which have greatly expanded our understanding of the biology of protein glycosylation. This review seeks to summarize these studies, illustrating that protein glycosylation, by modulating numerous biological attributes, is of central import in defining the utility of recombinant therapeutics.