Children's theory of mind (ToM) is typically measured with laboratory assessments of performance. Although these measures have generated a wealth of informative data concerning developmental progressions in ToM, they may be less useful as the sole source of information about individual differences in ToM and their relation to other facets of development. In the current research, we aimed to expand the repertoire of methods available for measuring ToM by developing and validating a parent-report ToM measure: the Children's Social Understanding Scale (CSUS). We present 3 studies assessing the psychometric properties of the CSUS. Study 1 describes item analysis, internal consistency, test-retest reliability, and relation of the scale to children's performance on laboratory ToM tasks. Study 2 presents cross-validation data for the scale in a different sample of preschool children with a different set of ToM tasks. Study 3 presents further validation data for the scale with a slightly older age group and a more advanced ToM task, while controlling for several other relevant cognitive abilities. The findings indicate that the CSUS is a reliable and valid measure of individual differences in children's ToM that may be of great value as a complement to standard ToM tasks in many different research contexts.