Duolingo is arguably the most popular language learning app available today, as measured by its metric of achieving 500 million downloads in 2020, yet remarkably little research has been conducted regarding its implementation or efficacy as a language-learning tool. The handful of studies that do exist have found positive outcomes, although methodologies vary so widely as to make comparisons or generalizations impossible. Researchers have noted, though, that Duolingo may be most beneficial as a supplemental tool rather than a stand-alone program. Given that many classroom learners are already familiar with and even use Duolingo, research into this kind of usage warrants further investigation. Therefore, the present study explores the possible effects of using Duolingo as a supplemental tool and investigates outcomes related to learners' fluency -- including lexical, oral, and written fluencies -- during a semester of beginning Spanish at the university level. In addition to linguistic outcomes, the students' attitudes and motivation during the semester are also examined. Data from these learners' semester-long use of Duolingo are compared to those of a control group using LearnSmart, an adaptive learning app provided by the textbook company McGraw-Hill. Furthermore, within the Duolingo users, half were assigned to a "competitive" condition, taking advantage of the program's game-like interface, in order to explore the benefits of gamified learning. Overall, Duolingo was shown to be advantaged over LearnSmart in terms of the learning outcomes, although not all differences were found to be significant. Despite this, the qualitative data overwhelmingly points to a preference for Duolingo, due to its fun, gamified and user-friendly design. These findings are discussed in terms of their implications for future research and careful implementation of apps in college language classrooms, taking into consideration features of each app prior to adoption. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]