Insight is a reliably measured construct that is stable across cultures, with several aspects assessed frequently. Insight impairment in schizophrenia appears to be more stable than in mania and tends to be worse at all stages than other psychoses or “at-risk states.” Good insight may lead to temporary low mood and poor self-image, but these processes are complex and perhaps not the same at different stages of illness. Depression and hopelessness mediate insight's relationship with suicidality. Insight predicts low self-rating of quality of life but better observer rating and social function. It did not predict violence in one large study but did in shorter-term studies of forensic or first-admission populations. First-episode studies find consistent links with relapse and readmission but weak evidence of insight predicting symptoms or function at follow-up. Atypical antipsychotics were not specifically beneficial in one large trial, but cognitive-behavioral therapy was in another.