Nanotechnology is a recent technology which is developing rapidly and it has a wide range of potential applications. It is the atomic-level tailoring of materials to achieve unique features that may be controlled for the intended purposes. Nanomaterials can be prepared via several physico-chemical methods but bioreduction of bulk to nanomaterials via green synthesis has developed as a viable alternative to physico-chemical methods in order to overcome their limitations. Plant-mediated nanomaterial synthesis has been found to be environmentally friendly, less costly, and safe with no use of chemicals for medicinal and biological applications where the nanoparticles purity is of major concern. Plant extract is used for the reduction of materials from bulk into nano scale instead of other toxic reducing agents used in chemical methods. The phytochemicals present the extract of plant not only facilitate the synthesis of nanomaterials but act as stabilizing and capping agent, also the shape and size of nanoparticles can be tailored by changing the nature and concentration of plant extract. The present chapter focuses on the green synthesis of nanoparticles mediated by various Brassica species and their potential medicinal and biological applications.