This study investigates the role of prior knowledge in supporting Japanese foreign language (FL) learners' L2 higher order thinking skills. Scholars recognize prior knowledge integration as the critical stage where learning occurs (Anderson et al., 2001; Britton, 1994; Kintsch, 1988, 1998), and consider it an untapped resource in the FL classroom (Bernhardt, 2005; Hulstijn, 2011; Koda & Yamashita, 2018). To this end, Koda & Yamashita (2018) developed the "reading to learn" framework, which entails the extraction of text information, integration of relevant prior knowledge with text content, and refinement of knowledge. Employing this framework, this study investigates the role of prior knowledge in promoting FL learners' "reading to learn" skills. Sixty-six Japanese as a foreign language learners took two versions of a "reading to learn" test, where they read about societal issues in Japan. One version provided scaffolding to help learners integrate relevant prior knowledge with the text, while the other version provided language activities. Students then expressed their refined understanding of the topic in a series of reflection questions, which were scored and coded for prior knowledge activation. Findings reveal that the scaffolding condition slightly increased learners' prior knowledge activation, but did not affect learners' reflection scores, which were mainly predicted by the response length. Although learners displayed adequate reading comprehension and coherent writing, poor scorers tended to not use information from the text or their prior knowledge to support their conclusions. These findings suggest that FL learners can be encouraged to utilize their world knowledge in class, but may lack the motivation or linguistic fluency to explain this knowledge when discussing complex cultural issues. Learners may benefit from integrated language and content learning activities, where they can obtain more practice with "reading to learn" and recognize it as a desirable learning outcome. [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.]