Most therapeutic antibodies are produced using either Chinese hamster ovary (CHO) or mouse NS0 (myeloma) cells. Despite their relative ease of use and extensive characterization, the use of mammalian cells for producing mAbs has shortcomings. The first demonstration of the successful expression of an IgG in plants was in 1989; since then, improvements in the production of mAbs by plants have contributed to realistic proposals for the use of plant systems as an alternative platform for the production of therapeutic proteins. Plant-based production of recombinant proteins using N. benthamiana is a safe, low-cost, and highly efficient alternative production platform to the standard CHO or NS0 cell culture systems for the production of high-value therapeutic proteins. This chapter illustrates the developmental stages involved in stable transgenic plant production. Improper glycosylation, such as the attachment of truncated N-glycans, causes mAbs to lose their quaternary open Y-shaped structure. The development of biosimilar drugs in plants requires a humanized N-glycosylation system.