Second language (L2) writing has been determined to be influenced by what is read, with language learners tending to borrow words and short phrases from input texts while writing (e.g., Wang & Wang, 2015). This phenomenon is referred to as lexical alignment. Only one empirical study has investigated the influence that the linguistic complexity of reading materials has on L2 written alignment. Peng et al. (2020) found that reading more linguistically complex materials resulted in significantly lower reading comprehension scores, yet it had no significant influence on lexical alignment. Peng et al.'s study also found that participants who had read the simpler version of a text tended to write more words than those who had read the more complex text. While researchers have hypothesized that cognitive individual difference variables may influence L2 written alignment (Wang et al., 2021), no research has empirically investigated the influence that working memory or language aptitude may have on L2 written alignment. This dissertation study builds on existing alignment research by investigating the influence that linguistic complexity of a read text has on L2 written alignment in an integrated writing task, as well as whether and how reading comprehension, working memory, and language aptitude covary and interact with what is read and what is written. Reading comprehension was measured using a 15-question multiple choice test. Working memory was assessed using a 75-item operation span test (Li & Roshan, 2019). Language aptitude was evaluated using individual LLAMA subtest scores (Meara & Rogers, 2019). One hundred thirty-four Colombian undergraduate students majoring in foreign language education were recruited from their English language courses. Participants were assigned to one of three groups using block random assignment to ensure that all groups shared similar English language proficiency. Treatment across groups varied by the lexical sophistication, lexical diversity, and syntactic complexity of the text they were assigned to read as part of an integrated writing task. One treatment group read a text that was modified to be linguistically similar to texts encountered in their English textbooks and participants in another treatment group read a simplified version of the text. The control group read a Spanish translation of the simpler text. Participants in all three groups completed a reading comprehension test and then wrote their responses to a prompt relevant to the reading topic. Participants' writing was evaluated for alignment, syntactic complexity, accuracy, lexical complexity, and fluency (CALF). Each of these components of participants' writing was compared using ANCOVA analyses with interaction effects to determine whether and how covariates influenced participants' L2 writing differently in the three conditions. Results indicated that significant alignment occurred in both treatment groups, with 98-99% of the words produced by participants also appearing in the text they had read. No significant between-group differences in alignment or CALF were uncovered. Reading comprehension was found to be a significant predictor of all three measures of accuracy employed in this study. Working memory significantly interacted with the grouping variable with higher working memory scores corresponding with higher alignment in the simple treatment condition and lower alignment in the complex treatment condition. LLAMA D, the aptitude subtest associated with listening for new words, had a significant and negative main effect on error rates in participants' writing, therefore positively influencing written lexical, morphosyntactic, and overall accuracy. LLAMA E by group interaction approaching significance was also uncovered pertaining to written fluency, indicating that participants with higher scores on the sounds and symbols aptitude test who had read the simple version of the test had significantly greater fluency than their peers who shared a similar aptitude score but read the text in Spanish, the participants' L1. LLAMA F, a measure of grammatical inferencing abilities, was a significant negative predictor of lexical and overall error rates. Findings are discussed with consideration for previous alignment research and theories of language learning. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]