"Thinking Reader" is a software program for students in Grades 5-8 that incorporates elements commonly identified in policy reports as being key components of effective adolescent literacy instruction. This evaluation of the impact of "Thinking Reader" use by Grade 6 students focused on two confirmatory research questions about the effect of the program on two measures of students' reading achievement: (1) What is the effect of "Thinking Reader" on students' reading vocabulary?; and (2) What is the effect of "Thinking Reader" on students' reading comprehension? A statistically significant impact on either outcome measure would signal the program's success. The study also examined whether "Thinking Reader" has an effect on two ancillary, but important, measures of students' approaches to reading: (1) What is the effect of "Thinking Reader" on students' use of reading comprehension strategies?; and (2) What is the effect of "Thinking Reader" on students' motivation to read? This study also addressed four exploratory research questions. These questions investigate whether the impact of the "Thinking Reader" intervention on students' reading achievement varied across subgroups of students formed on the basis of baseline reading vocabulary, baseline reading comprehension, and baseline motivation to read measures: (1) Does the effect of "Thinking Reader" on students' reading vocabulary vary according to their baseline reading vocabulary scores?; (2) Does the effect of "Thinking Reader" on students' reading comprehension vary according to their baseline reading comprehension scores?; (3) Does the effect of "Thinking Reader" on students' reading vocabulary vary according to their baseline reading motivation scores?; (4) Does the effect of "Thinking Reader" on students' reading comprehension vary according to their baseline reading motivation scores? The impact results for the primary research questions indicate that "Thinking Reader" was no more effective than business as usual in improving students' reading vocabulary (effect size of -0.04) or reading comprehension (effect size of 0.03). Results for the ancillary research questions indicate that "Thinking Reader" was also no more effective than business as usual in improving student' use of reading comprehension strategies (effect size of 0.03) or their motivation to read (effect size of -0.03). None of these results are statistically significant. Sensitivity analyses found no changes in the direction or magnitude of the intervention effects. Appendices include: (1) Examples From the "Thinking Reader" Program; (2) Data Collection; (3) Missing Data, Baseline Equivalence of the Analytic Sample, and the Impact Model; (4) Sensitivity Analyses; and (5) Exploratory Analyses. (Contains 89 tables, 10 figures, 6 boxes, 7 exhibits and 51 footnotes.)