In 1973, Landauer and Streeter (1973) introduced N-metric. N refers to the number of words that can be created by changing a single letter of a target word. One of the first studies in this field was conducted by Coltheart, Davelaaar, Jonasson, and Besner (Coltheart, Davelaar, Jonasson, & Besner, 1977), who reported that classification of high-N nonwords is slower than that of low-N nonwords. Since 1973, few studies have actually measured neighborhood effect on foreign language learners' lexical retention. The aim of this book is to move beyond first language and apply current lexical retention models and methods on second language lexical retrieval.