We hear a lot about “digital health” these days. As data about our health piles up — thanks to sources like electronic health records, personal fitness apps and gadgets, and home genome test kits — we should understand a lot more than we used to about what’s wrong with our health and what to do about it. But having a lot of data is not enough. We have to be aware of what we have, understand what it means, and act on that understanding. While the challenges are in some ways more acute in the United States because of its fragmented system of care, they exist in health care across the globe.