This book presents a comprehensive review of previous research on lexical inferencing, co-authored by Kirsten Haastrup, and a major new trilingual study of lexical inferencing by both first (L1) and second language (L2) readers. Research since the 1970s on this apparently universal cognitive process in L2 reading and vocabulary learning is surveyed, including the kinds of knowledge and textual cues L2 readers use when inferring unknown word meanings, factors influencing their success and knowledge retention, and relevant theory. A comparative study of L1 and L2 lexical inferencing by Persian and French and English speakers is then presented, focusing on evidence of L1 transfer in the L2 inferencing process, its success and readers' gains in L2 word knowledge. Influences of the specific L1 are distinguished from those of native versus non-native proficiency, relative cultural familiarity of texts, readers' L2 proficiency, text language features and other factors. The relative typological distance between readers' L1 and L2 is reflected in systematic differences between L1 speakers of Persian and French in their L2 lexical inferencing. Implications are drawn for L2 instruction at advanced levels.