The Common Core State Standards emphasize the value of teaching students to engage with complex text. But what exactly makes a text complex, and how can teachers help students develop their ability to learn from such texts? The authors of this article discuss five factors that determine text complexity: vocabulary, sentence structure, coherence, organization, and background knowledge. Teachers should not shrink from requiring their students to tackle complex text, the authors assert; rather, they should support students in mastering such text by building vocabulary and fluency, establishing a purpose for reading, and fostering motivation and persistence. (Contains 1 endnote.)