In the current study, we address calls for research on the complex nature of integrations of pedagogical content knowledge (PCK) components. This is a multiple case study of six middle school pre-service teachers (PSTs) as they taught science in their school practicum. We investigated the nature of PSTs' integration between knowledge of students' understanding (KSU) and instructional strategies (KIS), and their sources of these integrations. The primary data sources were two video stimulated recall interviews during which each PST viewed video recordings of their instruction, and shared reflections on their teaching. Results were represented as PCK maps. The PSTs frequently demonstrated integration of KSU and KIS, often developing topic-specific strategies. Instructional strategies served a variety of goals in response to students' needs. PSTs referred to specialized science content courses, peer PSTs, learning experiences, and mentor teachers as sources that contributed to the integrations. Implications for research and teacher education are included.