Extensive reading (ER) has been implemented under a wide variety of names, such as "(uninterrupted) sustained silent reading," "free voluntary reading," "pleasure reading," "book flood," "independent reading," and "Drop Everything And Read (DEAR)"; its various names emphasize different aspects of the same/similar kind of reading. Since a series of pioneering publications by Elley and his colleagues demonstrated ER's positive impacts on second language (L2)1 learning in various countries (e.g., Elley, 1991, 2000; Elley & Mangubhai, 1983), ER has received attention as a possible and promising way of improving L2 abilities. In recent years, there has been phenomenal growth in the interest in ER amongst L2 researchers and educators. Conferences, professional associations, and journals specifically focusing on the theme of ER have been founded (e.g., the World Congress on Extensive Reading, Japan Extensive Reading Association, JALT ER Special Interest Group, "Extensive Reading in Japan," "Journal of Extensive Reading"). In addition, a number of online resources to help promote ER in educational settings are now available (cf. Lynn, 2014). Given this situation, this paper is an attempt to contribute to our understanding of the fundamentals of ER and to clarify its nature.