The testing effect is a finding from cognitive psychology with relevance for education. It shows that after an initial study period, taking a practice test improves long-term retention compared to not taking a test and--more interestingly--compared to restudying the learning material. Boundary conditions of the effect that have received attention include the test format, retrieval success on the initial test, the retention interval, or the spacing of tests. Another potential boundary condition concerns the complexity of learning materials, that is, the number of interacting information elements a learning task contains. This insight is not new, as research from a century ago already had indicated that the testing effect decreases as the complexity of learning materials increases, but that finding seems to have been nearly forgotten. Studies presented in this special issue suggest that the effect may even disappear when the complexity of learning material is very high. Since many learning tasks in schools are high in element interactivity, a failure to find the effect under these conditions is relevant for education. Therefore, this special issue hopes to put this potential boundary condition back on the radar and provide a starting point for discussion and future research on this topic.