Task-based language teaching and interaction are facilitative contexts for instructed second language acquisition and include abundant opportunities for learners to receive authentic input, engage in negotiation for meaning and pushed output, and to receive feedback and direct attention to linguistic features (Goo, 2019, p. 247). Individual differences such as willingness to communicate (WTC) may mediate opportunities for interaction, the amount of interaction that learners engage in, and resulting L2 acquisition. Research on WTC indicates that learners with higher levels of trait WTC may interact more and, as a result, improve their oral proficiency at a faster, greater rate than their counterparts with lower WTC (Robson, 2015) and that WTC may change because of the personal relevance that topics have to learners (MacIntyre & Wang, 2021), among other reasons. Research on task-based interaction in laboratory studies finds that the presence of acquisitionally-relevant factors like language-related episodes may encourage L2 development (Baralt, Gurzynski-Weiss, & Kim, 2016) and that interactional features (e.g., modified output) have been linked to learners noticing feedback (Gurzynski-Weiss & Baralt, 2015). The goal of this dissertation study is: (1) to extend research on task-based interaction and individual differences, (2) to examine trait WTC, task-specific WTC, and classroom WTC during seven tasks scaffolded to increase in complexity, and (3) to explore possible relationships between task-based interactional features (language-related episodes and type of modified output), trait WTC, task-specific WTC, classroom WTC, and Complexity, Accuracy, Fluency, and Functional Adequacy (CAFFA) measures. The study quantitatively measures WTC from 24 L2 Spanish students participating in a classroom study at a small liberal arts university in the Midwest to investigate: (1) the dynamicity of trait, classroom, and task-specific WTC, (2) how task-specific and classroom WTC relate to task-based interaction, and (3) how measures of WTC and task-based interaction relate to task-based outcome measures (CAFFA). This dissertation contributes to the fields of instructed second language acquisition, task-based language teaching, and individual differences. While previous work has examined WTC as an individual differences as well as task-based interaction, no previous studies have examined these variables together longitudinally to identify how these variables interact with one another ultimately impacting L2 development. In sum, this dissertation provides an in-depth analysis of WTC and task-based interaction and how they may work together to mediate L2 development. [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.]