Across nearly five hundred studies and six continents, researchers have attempted to draw an honest map of what a smartphone can and cannot do for human health. The findings reveal a technology that genuinely helps where behavior and monitoring intersect — weight, blood glucose, mental wellbeing — but falls silent before the deeper metabolic and lifestyle transformations that apps alone cannot compel. It is a portrait of a useful but bounded tool, a mirror that can show us our habits without always having the power to change them.