The Cotwin Control Method is one of the most fascinating experimental designs in developmental and educational studies. Its methodological features and difficulties were, discussed and 8 studies ever done were reviewed. Especially, Gesell  Thompsons' study (1929), which is famous for maturation theory, and the critics against their study were examined in detail. A new explanation of the concept "maturation" was introduced. Finally, the overall formula of twin study from the view point of modern behavioral genetics was shown, and the author's cotwin-controlled instructional studies were introduced.