ABSTRACT To read, a reader must know words. To read better, a reader must learn more words. Although this relationship appears axiomatic, the acquisition of full conceptual knowledge of words is a deceptively complex process. This article summarizes five theories that have been set forth to explain the relationship between word knowledge and comprehension of text in L1. In addition, an overview of recent research that has demonstrated this relationship is presented. From these two sources, L1 researchers have begun to identify the characteristics of effective vocabulary instruction. This article synthesizes these findings and explores possible implications for L2 researchers and practitioners. Finally, two classroom applications of the knowedge hypothesis are delineated.