Science tells us that medical screening sometimes saves lives — but can also lead to overdiagnosis, overtreatment and medical harm. Screening can produce false positives, finding disease that isn’t there; false negatives, missing signs of very real disease; or ambiguous information that confounds patient and doctor alike. Pap smears, mammograms, and PSA tests are among the screenings routinely administered in the US and in Europe in ways that contradict national recommendations. Full-body scans have become popular, but no medical entity recommends them for a healthy population. Genetic screening is an ethically complex minefield. What’s the risk of too much screening? Of too little?