This discussion piece continues the discussion forum on extensive reading (ER) from the April 2015 issue of "Reading in a Foreign Language." In that forum, a number of the discussions were concerned with the principles of ER (Day & Bamford, 2002) in implementing ER. Our discussion also concerns the principles; we examine ER programs that used what we believe are five core principles and their impact on second language (L2) reading proficiency. The aim is to show how much effect we can expect from ER programs on L2 reading proficiency through the tool of meta-analysis. We discuss the results of a meta-analysis that was done to determine the effect of ER programs on L2 reading proficiency in two settings, English as a foreign language (EFL) and English as a second language (ESL). These ER programs all used the five core principles. For details on the meta-analysis and the study, see Jeon and Day (in progress).