This article presents evidence from a large-scale randomized controlled trial of the effects of four supplemental reading comprehension curricula (Project CRISS, ReadAbout, Read for Real, and Reading for Knowledge) on students' understanding of informational text. Across 2 school years, the study included 10 school districts, more than 200 schools, and more than 10,000 fifth-grade students. Schools interested in implementing 1 of the 4 supplemental curricula were randomly assigned to 1 of 4 treatment groups or to a control group. The impact analyses in the study's first year revealed a statistically significant negative impact of Reading for Knowledge on students' reading comprehension scores and no other significant impacts. The impact of ReadAbout was positive and significant in the study's second year among teachers with 1 year of experience using the intervention. (Contains 5 footnotes and 11 tables.)