Two decades of international research in applied linguistics provides a large number and variety of topics from which to choose for this special anniversary edition, but certainly one of the most significant among these choices is the critical period hypothesis (CPH). Few topics in applied linguistics have continued to captivate the interests of researchers and practitioners so intensively and for such a long period of time as the CPH. Indeed, one could easily go back to reviewing three, not two decades of sustained research and continuous interest in this topic (Lenneberg 1967, Scovel 1969). If number and diversity of publications is indicative, the CPH has engendered even more interest and controversy now than in any previous decade. Why is this so?