<p>The dissertation is a study of the efficacy of reading materials for learners of Japanese as a foreign language (JFL). It discusses the merits of 'authentic' materials written primarily for native speaker-readers compared to 'modified' texts adapted in some way for learners. Further, it compares various sorts of modifications: simplification, elaboration, marginal glosses and the use of onscreen computer pop-ups. More broadly, it locates the study within the wider discourse of pedagogy concerning reading materials for second language learners, especially JFL learners.  Reading in Japanese as a second language is generally thought to be more demanding than reading in some other second languages. The study therefore argues that the authenticity debate and efficacy of text modification must be addressed specifically in the JFL reading pedagogy.  In the context of the authenticity debate, there are, broadly, two opposing views. One favours the predominant use of unmodified texts while the other promotes the efficacy of modified texts. While there have been numerous theoretical discussions and empirical findings in the reading pedagogy of English as a second or foreign language (ESL/EFL), the JFL reading pedagogy is currently lacking such academic endeavours. Hence, the present study seeks to fill the gap.  The study is mixed methods research, consisting of three projects in which both qualitative and quantitative methods are employed. This approach investigates equally the effects of text modification on participating learners' cognitive changes (reading comprehension) and affective changes (motivation and perception).  The results indicate that learners of Japanese comprehend modified texts statistically significantly better than they do unmodified texts. Findings include that modified texts for Japanese are more efficacious than they are in the ESL/EFL context. However, modified texts that are insufficiently challenging fail to enhance learners' motivation. Advanced learners especially were found to have a negative attitude toward reading modified Japanese texts.</p>