In this commentary, the author reflects on four studies that have greatly expanded the knowledge base on effective interventions in mathematics, and he provides four rigorous experimental studies of approaches for students likely to experience difficulties learning mathematics over a large grade-level span (pre-K to 4th grade). All of the interventions appear to be implemented with care and rigor, and data were analyzed using state-of-the-art techniques. The studies all demonstrated positive, statistically significant effects on measures aligned with the instructional goals of the intervention curriculum--what authors tend to call "proximal measures." The finding suggests that not only students who score close to benchmark levels, but even the lowest-performing students in mathematics, do benefit from a well-designed Tier 2 mathematics intervention covering difficult topics in the grade-level curriculum. This is an important finding, given finite money and resources.