Executive function deficits have been reported in both autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD). However, little is known regarding which, if any, of these impairments are unique vs. shared in children with ADHD versus ASD. In this Review, we provide an overview of the current literature with a critical eye toward diagnostic, measurement, and third-variable considerations that should be leveraged to provide more definitive answers. We conclude that the field's understanding of ASD and ADHD executive function profiles is highly limited because most research on one disorder has failed to account for their co-occurrence and the presence of symptoms of the other disorder; a vast majority of studies have relied on traditional neuropsychological tests and/or informant-rated executive function scales that have poor specificity and construct validity; and most studies have been unable to account for the well-documented between-person heterogeneity within and across disorders. Currently, the most parsimonious conclusion is that children with ADHD and/or ASD tend to perform moderately worse than neurotypical children on a broad range of neuropsychological tests. However, the extent to which these difficulties are unique vs. shared, or attributable to impairments in specific executive functions subcomponents, remains largely unknown. We end with focused recommendations for future research that we believe will advance this important line of inquiry.