Assessment of adult psychopathology relies heavily on self-reports. To determine how well self-reports agree with reports by "informants" who know the person being assessed, the authors examined 51,000 articles published over 10 years in 52 peer-reviewed journals for correlations between self-reports and "informants" reports. Qualifying correlations were found in 108 (0.2%) of the articles. When self-reports and informant reports were obtained with parallel instruments, mean cross-informant correlations were .681 for substance use, .428 for internalizing, and .438 for externalizing problems. When based on different instruments, the mean cross-informant correlation was .304. The moderate sizes of the correlations argue for systematically obtaining multi-informant data. National survey findings were used to illustrate practical ways to obtain and use such data.