This article reviews the use of third party information and its potential to increase the accuracy and usefulness of child custody evaluations. Legal and admissibility issues are reviewed, along with the types of third party information commonly utilized. Factors impacting such data sources are discussed, as well as the importance of determining the degree to which independent sources point to the same conclusion. Suggestions are provided for selecting, contacting, and interviewing collateral sources, and ways of assessing, utilizing, and reporting collateral data are described.