This dissertation analyzes the development of adult second language (L2) English learners’ depth of lexical knowledge over six months through a series of three longitudinal experimental studies. The goal of the project is to provide a better understanding of how the English lexicon develops over time in L2 learners. Methods employed include vocabulary size assessment, a word association task, lexical decision semantic priming, and the computational analysis of subjects’ spoken lexical output. Results found little evidence of longitudinal development in L2 subjects’ lexical network knowledge, with the exception of L2 word associations, which actually became less native-like over time. In addition, results indicated no observed relationship between vocabulary size and the dependent measures obtained in the three studies. This dissertation project has theoretical implications for our understanding of depth of lexical knowledge and its rate of development. The project also has methodological implications for future experimental and/or longitudinal investigations of the L2 lexicon.