Molecular farming technology offers a unique advantage that almost any protein can be produced economically and safely under very controlled conditions. Besides traditional production systems, such as bacteria, yeasts, insects and mammal cell lines, plants can now be used to produce eukaryotic recombinant proteins, especially therapeutic ones. Their advantages as hosts for protein production include correct post-translational modifications, low-cost maintenance and no risk of contamination by human pathogens. The system is widely applied in agriculture and industry, especially in life science and pharmaceutical industry. The application of transgenic plants in the production of vaccines, antibodies and pharmaceutical proteins has been playing a key role in plant genetic engineering in recent years. The production of recombinant proteins plays a critical role in the production of high amounts of high-quality proteins. In this review, common problems in the production of recombinant proteins and antimicrobial peptides in plant-based expression systems are discussed and strategies for their solution are suggested. Viral vector-mediated transient gene expression in plants enables rapid production of pharmaceutical proteins such as vaccine antigens and antibodies. To conclude, plant-based systems have the potential to bring unique efficacy-enhancing features to increase the utility and effectiveness of vaccines and therapeutics.