Problem behavior of 52 children with Williams syndrome ages 6 to 17 years old was examined based on both parent and teacher report. Generally good inter-rater agreement was found. Common areas of problem behavior based both on parent and teacher report included attention problems, anxiety difficulties, repetitive behaviors (e.g., obsessions, compulsions, picking nose or skin), and social problems, reflecting a robust behavioral phenotype in Williams syndrome present across contexts. Some rater differences were observed; most notably, parents reported more attention and mood difficulties than did teachers, while teachers reported more oppositionality and aggression than did parents. Relations to intellectual functioning, age, and gender were examined. The implications of the findings for understanding the behavioral phenotype associated with Williams syndrome are discussed.