Plants have been used as medicines by humans for several millennia by now. Plant molecular farming refers to the extraction and use of specific compounds from plants. The latter can be wild-type or genetically engineered. For example, plants contain small molecules with therapeutic activity such as paclitaxel that is used in breast cancer treatment. Likewise, plants and plant cells can produce functional recombinant proteins such as antigens and antibodies as well as enzymes, growth factors, and hormones, which represent the active pharmaceutical ingredients of many medicines. Here, an overview is provided about the plant species often used for recombinant protein expression, the respective expression strategies, and potential modifications of the hosts to improve product accumulation and activity. The downstream process development is discussed in terms of unit operations and recovery. Furthermore, the specific applications of plant-based therapeutic protein expression such as oral delivery with minimal processing and a sustainable, cascading use of the residual biomass are highlighted.