Arabidopsis thaliana has been widely used in studies on basic plant physiology and biochemistry as well as in plant molecular genetic manipulations and developmental biology research because of its small genome, low chromosome number, short regeneration time (4–6 wk), availability of many mutants and genetic maps, sexual self-compatibility, and prolific seed production. More extensive use of Arabidopsis has been hampered because of difficulties in efficient and rapid regeneration and transformation procedures. Several methods for plant regeneration have been reported (1–7). Transformed plants have been recovered from various explants, such as leaf (8), stem (9), callus tissue (10), germinating seeds (11), root (12), and by using direct gene transfer to protoplasts (13). Despite these reports, the frequency of regeneration of transgenic plants was still low and took at least a few months to produce transgenic plants. Also the long period of in vitro incubation during shoot regeneration in these methods may increase the possibility of somaclonal variation and increases in ploidy level. Many reports have indicated the high regeneration potential of cotyledon explants at various stages of development in maturing embryos or after seed germination (14–17). This chapter presents a new procedure for rapid and prolific regeneration of shoots from cotyledon explants of Arabidopsis in ecotypes “Landsberg erecta” and “C24.” Furthermore, this regeneration procedure is developed establishing a method for rapid production of transgenic Arabidopsis shoots using disarmed Agrobacterium tumefaciens within 2–3 wk (18–20). A transformation procedure using root explants is also presented as a separate method.