Social problems are often given a diagnosis. A medical diagnosis can have value if it provides new insights or helpful actions. People often worry about disease underdiagnosis, but O’Sullivan argues that overdiagnosis is a greater current problem, where medical diagnoses increasingly may do more harm than good. Autism and ADHD diagnoses have risen for 30 years and increasingly extend beyond the severe consequences first identified in diagnostic guidelines. It is pertinent to question if diagnoses are extended too far if severe conditions appear stable and 85% of current diagnoses reflect ‘mild’ symptoms. There is little evidence that diagnoses have positive impacts for patients, and diagnosis may make people sicker and more convinced that they suffer from insurmountable problems. Increased screening can identify more signals for potential future disease, even when no treatment will ultimately be required. Overdiagnosis often leads to overmedicalization, where common behaviors are treated as medical problem, or invasive treatments may be applied when not warranted. More healthcare does not mean better health, and estimates suggest a shockingly high share of interventions with no benefits for patients or society. Testing more children for autism increases waiting lists and makes access to help more difficult for people who really need it. This thoughtful book is a useful guide to limitations in diagnosis, more realistic expectations of what medicine can do, and for better debates on policy and responses to public health.