Despite the importance of prior knowledge to learning, few studies have systematically addressed the extent to which or ways in which instructional resources support teachers in the activation of students' prior knowledge, or the extent to which teachers prompt students' prior knowledge activation in practice. In study one, a content analysis was applied to the language in four instructional resources to come to an understanding of the ways in which resources guide teachers to activate students' prior knowledge. In study two, we went beyond teacher resources to examine prior knowledge activation in four (two male, two female) public elementary teachers' reading classrooms. A two-step procedure was followed to analyze teachers' utterances, first discriminating language intended to activate students' prior knowledge (n = 27) from other utterances (n = 1,770), and then examining the 27 knowledge activation utterances and scoring them according to the frame of reference. The results suggest that instances of prior knowledge activation were rare during reading lessons, both with recommended statements and demonstrated actions. Additionally, the analysis found that instructional resources focused predominantly on world knowledge and personal experience, whereas the practitioners focused on knowledge from previously taught lessons.