In the past two decades, advances in plant molecular biology and recombinant antibody engineering have provided new tools for plant biotechnology. Plants are emerging as a safer alternative to microbial or animal cells for the production of pharmaceutical proteins, such as antibodies. Functional antibody expression in transgenic plants was demonstrated (Hiatt et al., 1989) and used to create virus-resistant plant lines (Tavladoraki et af, 1993). The progress and potential of this approach for virus control is reviewed in the First part of this chapter. Furthermore, plant-produced antibodies have been tested in immunotherapy (Ma and Hein, 1995b). Here, plant viruses as vectors for transient gene expression offer an alternative to transgenic plants. The development of this new system for the production of recombinant proteins, such as antibodies. in plants is the focus of the second part of this chapter, followed by a brief comment on prospects of the presented techniques.