This study examined associations between sleep problems and attentional and behavioral functioning in 137 children aged 7 to 13 years with anxiety disorders (n = 39), attention deficit hyperactivity disorder (ADHD; n = 38), combined anxiety disorder and ADHD (n = 25), and 35 controls. Diagnoses were made using the semistructured diagnostic interview Schedule for Affective Disorders and Schizophrenia for School-age Children-Present and Lifetime Version. Sleep problems were assessed using the Children's Sleep Habits Questionnaire, attention was measured by the Attention Network Test, and behavioral problems were measured by teacher ratings on the Achenbach System of Empirically Based Assessment, Teacher Report Form. Sleep problems were associated with reduced efficiency of the alerting attention system for all children and with increased internalizing problems in children with anxiety disorders.