Discrepancies are often found between self-reports and reports by others regarding psychopathology. Both the person being assessed and various informants may contribute crucial data concerning a person's functioning. Comprehensive assessment requires data from multiple informants. Such data can be easily obtained with parallel self-report and collateral-report forms. The multi-informant data can be compared, aggregated, and used in many ways. Optimal use of multi-source data is essential for clinical assessment and for discovering causes and cures of psychopathology.