Expression and purification of recombinant proteins produced in plants is emerging as an affordable alternative to using more costly mammalian bioreactors since plants are capable of producing mammalian proteins at high concentrations. There are two general methods of expressing foreign proteins in plants, namely, transient expression and stable transgenic expression. Both methods have advantages which serve different purposes. Nicotiana benthamiana is primarily used as plant host for transient expression of foreign proteins. This system is capable of producing high yields of antibody in a relatively short period of time (days); however, intensive upstream processing is required as each plant must be infected with Agrobacterium tumefaciens cells by vacuum infiltration. N. tabacum is often used for production of stable transgenic plants through a procedure that requires longer development time (months). Although antibody yields are smaller compared with the transient method, the advantage of using stable transgenic expression is that very little upstream process management is required once homozygous seed lines are developed. In this chapter, we describe the basic methodologies for expressing antibodies in plants using the transient and transgenic systems.