Recombinantly produced therapeutic proteins bring benefits to patients and production companies alike. However, due to high production costs the potential of this technology cannot be fully tapped and therefor safe, scalable, and economic alternatives to the standard mammalian cell culture-based manufacturing systems are needed. Plant-based expression systems with their current technological advances constitute such an alternative. Many recombinant biopharmaceuticals are glycoproteins and their structural properties and pharmacokinetics are strongly influenced by their glycosylation profile. Differences in glycosylation between plants and mammals can for this reason result in different therapeutic efficacies. In particular, low levels of sialylation may lead to a short serum half-life of therapeutic proteins and nonhuman types of glycosylation can induce degradation and immunogenic responses. Controlling glycosylation of plant-derived therapeutics is therefore fundamental to enhance their efficacy and eliminate possible adverse effects caused by non-authentic glycosylation. Here we describe methods to transiently express high levels of recombinant proteins in Nicotiana benthamiana and simultaneously modulate their glycosylation pattern towards the synthesis of highly sialylated humanlike structures.