Plants offer a number of attractive benefits over conventional mammalian or bacterial cell culture systems for the production of valuable pharmaceutical and industrial proteins. Currently, antibodies and their derived fragments represent the largest and most important group of biotechnological products in clinical trials. In particular, single-chain antibodies are an interesting class of biopharmaceuticals because they are able to overcome specific problems associated with full-length antibodies. Another valuable antibody format is the scFv-Fc: fusion of the Fc domain to a single-chain variable fragment restores antibody effector functions, allows purification, and mimicks, despite being a ‘single-gene’ product, the bivalent properties of a full-length IgG. Although many different plant-based production platforms have been evaluated for antibody production, seeds are especially attractive because, as natural storage organs, they provide an optimal biochemical environment for the accumulation and long-term storage of large amounts of functional proteins. This chapter describes how to achieve high-level seed-specific expression of antibody fragments, how to select the best transgenic lines, and how to evaluate the accumulation level in the seed stocks from the selected lines.