This chapter describes the technical aspects, advantages, problems and bottlenecks of plant factory. In plant factory, plants provide an alternative source for the production of a range of valuable proteins, such as biopharmaceuticals. Among higher eukaryotics, plants have been found to be the best biological system for reliability and ability in expressing, assembling and modifying proteins. Using existing infrastructures for the growing, processing and storage of crops, plants offer a flexible production system for producing a theoretically unlimited quantity of all the biopharmaceuticals that humans need, now and in the future, at a relatively low cost. The fact that plants can not host harmful human pathogens represents a substantial added value, and in addition plants could be used for directly delivering vaccines. Bottlenecks are principally the need to demonstrate cost-effectiveness from both a technical and a regulatory point of view and the challenge of solving potential environmental and social concerns.