This article describes a first attempt to investigate the reliability and validity of the TOM test, a new instrument for assessing theory of mind ability in normal children and children with pervasive developmental disorders (PDDs). In Study 1, TOM test scores of normal children (n = 70) correlated positively with their performance an other theory of mind tasks. Furthermore, young children only succeeded on TOM items that tap the basic domains of theory of mind (e.g., emotion recognition), whereas older children also passed items that measure the more mature areas of theory of mind (e.g., understanding of humor, understanding of second-order beliefs). Taken together, the findings of Study 1 suggest that the TOM test is a valid measure. Study 2 showed for a separate sample of normal children (n = 12) that the TOM test possesses sufficient test-retest stability. Study 3 demonstrated for a sample of children with PDDs (n = 10) that the interrater reliability of the TOM test is good. Study 4 found that children with PDDs (n = 20) had significantly lower TOM test scores than children with other psychiatric disorders (e.g., children with Attention-deficit Hyperactivity Disorder; n = 32), a finding that underlines the discriminant validity of the TOM test. Furthermore, Study 4 showed that intelligence as indexed by the Wechsler Intelligence Scale for Children was positively associated with TOM test scores. Finally, in all studies, the TOM test was found to be reliable in terms of internal consistency. Altogether, results indicate that the TOM test is a reliable and valid instrument that can be employed to measure various aspects of theory of mind.