In an earlier issue of Ideas & Perspectives, we offered “ISM’s 20 Success Predictors for the 21st Century.” As that article explained, ISM expects successful 21st Century Schools to make radical changes in both structure and function to achieve and sustain stability and excellence. We emphasized that the 20 Success Predictors were designed as speculative forecasts of what ISM expects to be needed to achieve long-term stability and excellence in upcoming years. Readers were also reminded that the current (third) iteration of the ISM Stability Markers®—the primary lens through which ISM views private-independent schools—does not lose its general utility as an evidence-driven set of benchmarks for long-term stability and programmatic excellence. Thus, ISM maintains its focus on the Stability Markers, but offers a future-focused set of Success Predictors as accompanying guidelines. (Note: There is some overlap between the two lists. Several of the ISM Stability Markers are also Success Predictors for the 21st century.) This article focuses on ISM Success Predictor No. 17: 3% of the overall budget allocated to professional development.