This book focuses on theory, research, and practice related to "lexical input processing" (lex-IP), an exciting field exploring how learners allocate their limited processing resources when exposed to words and lexical phrases in the input. Unit 1 specifies parameters of lex-IP research among other levels of input processing as well as key components (form, meaning, mapping) and contexts (incidental/intentional) of vocabulary learning. Unit 2 highlights theoretical advances, such as the type of processing -- resource allocation (TOPRA) model, consistent with research on tasks (sentence writing, word copying, word retrieval) that learners may perform during vocabulary learning. Unit 3 highlights patterns in partial word form learning and input-based effects, including the value of increased exposure, drawbacks of presenting vocabulary in semantic sets, and advantages of input enhancement, particularly with regard to increasing talker, speaking-style, and speaking-rate variability in spoken input. The book unifies a range of research pertinent to lex-IP, summarizes theoretical and instructional implications, and proposes intriguing new directions for future research.