Two studies examined the psychometric properties of the Theory of Mind Inventory (ToMI). In Study One, 135 caregivers completed the ToMI for children (ages 3 through 17) with autism spectrum disorder (ASD). Findings revealed excellent test-retest reliability and internal consistency. Principle Components Analysis revealed three subscales related to the complexity of ToM understanding. In Study Two, data were collected for 124 typically developing children (2 through 12 years). Findings again revealed excellent test-retest and internal consistency. The ToMI distinguished groups by age (younger vs. older children) and developmental status (typically developing vs. ASD), and predicted child performance on a ToM task battery. Utility of the ToMI, study limitations and directions for future research are discussed.