Learning new vocabulary words in a second language is a challenge for the adult learner, especially when the second language writing system differs from the first language writing system. According to the lexical quality hypothesis (Perfetti & Hart, 2001), there are three constituents to word-level knowledge: orthographic, phonological, and semantic. A set of studies investigated the nature of orthographic knowledge in advanced learners of English as a second language. In a data mining study, students' spelling errors were analyzed. Results showed that first language background and second language proficiency have an effect on the rates and types of spelling errors made. In two training interventions, students showed learning gains from two different types of spelling instruction: a form focus condition and a form-meaning integration condition (Norris & Ortega, 2000). In a separate audio dictation task, non-native English speakers were shown to be sensitive to word frequency and age of acquisition but not regularity. In a cross-modal matching task, the same students were most susceptible to transposition foils that preserved target letters but in an incorrect order, and least susceptible to phonological foils that preserved phonological but not orthographic form of the target word. In a spell checking task, students had more difficulty rejecting misspelled words that maintained the phonological form of the target word than misspelled words that did not preserve phonology of the target. Overall, findings suggest that intermediate to advanced learners of English as a second language still show difficulty with the language's deep orthography, but that they can benefit from minimal amounts of instruction. Furthermore, these students appear to be acquiring orthographic knowledge via exemplar-based rather than rule-based strategies. This research expands upon the lexical quality hypothesis and finds support for the arbitrary mapping hypothesis. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]