In this study we examine the interplay between curriculum-embedded formative assessment--a well-known teaching practice--and general features of classroom process quality (i.e., cognitive activation, supportive climate, classroom management) and their combined effect on elementary school students' understanding of the scientific concepts of floating and sinking. We used data from a cluster-randomized controlled trial and compared curriculum-embedded formative assessment (17 classes) with a control group (11 classes). Curriculum-embedded formative assessment and classroom process quality promoted students' learning. Moreover, classroom process quality and embedded formative assessment interacted in promoting student learning. To ensure effective instruction and consequently satisfactory learning outcomes, teachers need to combine specific teaching practices with high classroom process quality.